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Pulse radiolysis investigations of solvated electrons Ulrich, Mary McKenney 1974

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PULSE RADIOLYSIS INVESTIGATIONS OF SOLVATED ELECTRONS BY MARY MCKENNEY ULRICH A. B. (Hons) Mount Holydke College, USA, 1966 M. Sc. Cornell University, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILSOPHY i n the Department of Chemistry We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1974 In presenting th i s thesis in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L i b r a r y shal l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th is thesis for s cho lar ly purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l i ca t ion of th i s thesis for f inanc ia l gain shal l not be allowed without my wri t ten permission. Department of The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date r ^ . ^ O . I ? ^ - i i -ABSTRACT The method of Cerenkov Reabsorption Spectroscopy has been used to measure the i n i t i a l r a d i a t i o n y i e l d of hydrated electrons. The amount of Cerenkov l i g h t that i s absorbed by concurrently produced r a d i a t i o n products i s measured, and can be r e l a t e d to the i n i t i a l y i e l d of the absorbing species through Beer's Law. The a p p l i c a t i o n of the law i s not straightforward because of a complicated s p a t i a l dependence of both the l i g h t source and the concentration of the absorbing species. The problem was solved by ( i ) using the integrated form of Beer's Law with a correction f a c t o r f o r the s p a t i a l dependence of both the l i g h t production and the concentration of absorbers and ( i i ) applying the d i f f e r e n t i a l form of the law d i r e c t l y to a n a l y t i c a l expressions f o r the two functions,, An i n i t i a l y i e l d , independent of pH, of 4.0 i 0.4 was determined f o r the hydrated electron. This value i s discussed i n r e l a t i o n to the various models f o r the r a d i o l y s i s of water, and i n r e l a t i o n to the concepts of presolvation scavenging and high concentration rate constants. The second part of t h i s t h e s i s deals with an attempt to detect c h i r a l i t y , either inherent or induced, on the part of solvated electrons through ( i ) t h e i r reaction with c h i r a l molecules or ( i i ) t h e i r i n t e r a c t i o n with polarized l i g h t . The rate of reaction of solvated electrons with v a r i o u s c h i r a l molecules was measured u s i n g the method of k i n e t i c l a s e r spectrophotometry. Computer a n a l y s i s of the decay curves gave no evidence f o r s p e c i e s o f d i f f e r e n t r e a c t i v i t y . T h e r e f o r e , i f c h i r a l s o l v a t e d e l e c t r o n s e x i s t t h e i r r a t e c o n s t a n t s i n these i n s t a n c e s must d i f f e r by l e s s than 5$ d e s p i t e scavengers chosen such t h a t the r a t e c o n s t a n t s had a s m a l l a c t i v a t i o n energy and a r e l a t i v e l y l a r g e p r e e x p o n e n t i a l f a c t o r . No s h i f t i n the plane o f p o l a r i z a t i o n as d e t e c t e d by crossed p o l a r i z e r s or d i f f e r e n c e i n the a b s o r p t i o n o f r i g h t and l e f t c i r c u l a r l y p o l a r i z e d l i g h t by s o l v a t e d e l e c t r o n s i n a c h i r a l s o l v e n t , a racemic s o l v e n t and a symmetric s o l v e n t doped w i t h c h i r a l molecules - i v -TABLE OF CONTENTS Page I. General Introduction 1 A. The inte r a c t i o n of high energy electrons 2 with matter 1. Emission losses 2 2. C o l l i s i o n a l losses 2 3. Tracks * 5 4. Dose rate and LET $ B. Radiolysis of water 6 C. The hydrated electron 10 I I . The Primary Ra d i o l y t i c Y i e l d of the Hydrated 16 E l e c t r o n A. Introduction 16 1. R a d i o l y t i c y i e l d s 16 2. The method of Cerenkov Reabsorption 26 Spectroscopy B. Experimental 32 1. The electron source - the Febetron 32 2. Materials 35 3. Physical lay-out 38* 4. Dosimetry 46 a. Faraday Cup 46 b. Calorimetry 47 5. Cerenkov l i g h t vs. depth measurement 60 C. Treatment of Data 70 1. The integrated form of Beer's Law 70 2. The d i f f e r e n t i a l form of Beer's Law 72 - V -D. Results and Discussion 77 1. The primary r a d i o l y t i c y i e l d 77 2 . Assumptions &4 a. Presolvation scavenging #4 b. High concentration: rate constants 91 3 . Further considerations 94 4. Significance 97 a. Value and pH dependence 97 b. In r e l a t i o n to other measurements 97 c. Hydration time and spur d i f f u s i o n 9# model I I I . On the C h i r a l i t y of Solvated Electrons 102 A. General Introduction 102 1. Possible o r i g i n s of o p t i c a l a c t i v i t y 102 2 . General experimental approach 107 B. The Reaction of Solvated Electrons with 109 C h i r a l Molecules 1. Introduction 109 a. Kinetic equations 109 b. The optimum system 110 2 , Experimental 113 a. Radiolysis c e l l 114 b. Detection system 11$ c. Materials 120 d. Modifications 122 C. Results and Discussion 124 - v i -a. Mandelic a c i d 124 b. Glutamic a c i d , disodium s a l t 127 c. Camphor 130 d. Chlorobutyric acid 133 C. The Interaction of Solvated Electrons 139 with Polarized Light 1. Introduction 139 a. Polarized l i g h t forms 139 b. Experimental approach 142 2 . Experimental 14$ 3 . Results 149 a. Crossed p o l a r i z e r experiments 149 b. Absorption of c i r c u l a r l y 150 polarized l i g h t 4 . Discussion 151 5 . Conclusions and suggestions f o r 154 further study References 156 - v i i -LIST OF FIGURES Page 1. A P i c t o r i a l Representation of the S p a t i a l 4 D i s t r i b u t i o n of Ionizations and Excit a t i o n s i n Water Following Passage of a 0.5 MeV Electron . 2. S i m p l i f i e d Diagram of a Hydrated Electron Showing 11 an Orientation of Polar Molecules about the Electron. 3. Electron Dose vs. Depth P r o f i l e i n Aluminum. 34 4. Radiolysis C e l l f o r Cerenkov Reabsorption 37 Spectroscopy Experiments. 5. Experimental Set-up f o r Cerenkov Reabsorption 39 Spectroscopy. 6. Transmission C h a r a c t e r i s t i c s of the Interference 40 Line F i l t e r . 7. Photodiode C i r c u i t r y . 43 A T y p i c a l Trace of P a r t i a l l y Reabsorbed Cerenkov 44 Light. 0.40M NO^ Solution. 9. A Typ i c a l Current P r o f i l e as Measured with a 4# Faraday Cup. 10. Calorimeters. 51 11. Cooling Curve and Log Plot f o r Calorimeter D. 54 12. T y p i c a l Cooling Curve and Plot f o r Calorimeters 55 A and 6. 13. Cooling Curve and Log Plot f o r Calorimeter C. 57 14 . Dose Deposited as a Function of Distance from the 59 Center of the Elec t r o n Beam. 15. C e l l f o r Cerenkov Intensity vs. Depth 61 Measurements. 16. Cerenkov Light-Depth and Dose Depth Curves. 63 17. Dose-Depth Curves f o r Electrons of Various 66 Energy. 18. Intensity of the Transmitted Cerenkov Light as 76 a Function of Depth. v i i i 19. Log I vs. fx Plots f o r Three of the Nitrate 76* Solutions with Additives as Indicated. 20. Simulation of CRS Data f o r Nitrate +1M NaOH 7 9 21. Log I vs. Lifetime f o r the Series of Perchloric 6*1 Acid Solutions. 22. Log I vs. fx Plots f o r a Series of Acetone #3 Solutions. 2 3 . Simulation of CRS Data f o r Nitrate + 1M NaOH 9 0 without Including the Presolvation Scavenging Factor. 24. Experimental Set-up f o r the Kinetic Laser 115 Spectrophotometry Experiments. 25. K i n e t i c Laser Spectrophotometry C e l l . 116 26. Tuned C i r c u i t to Remove High Frequency S i g n a l . 122 27. a) The Decay Curve of the Hydrated Electron 125 i n a 0.55M Solution of 1-Mandelic Acid. b) F i r s t - o r d e r Kinetic Plot of the Above Decay. 2 8 . A Typical Decay Curve of the Solvated Electron i n 134 an Act. Amyl Alcohol Solution which was O.O865 M ( - ) - 3 - C h l o r o b u t y r i c Acid. 29. T y p i c a l First-Order K i n e t i c Plots f o r the Reaction 135 of the Solvated Electron with (-)-3-Chlorobutyric Acid. 3 0 . First-Order Decay Plot f o r the Solvated E l e c t r o n 136 i n an n-Amyl Alcohol Solution of dl-3-Chlorobutyric Acid. Insert: the Actual Decay Curve. 31. The Relationship Between Optical Rotatory Disper- 141 sion and C i r c u l a r Dichroism. 32. Experimental Set-up f o r P o l a r i z e r Experiments I46 3 3 . Light Transmitted as a Function of Po l a r i z e r I47 Setting. - i x -LIST OF TABLES I. Approximate Time Scale of Events i n Liquid 7 Water Following Passage of a 1 MeV Electron. I I . Rate Constants f o r the Reaction of Primary 9 Species i n the Radiolysis of Water. I I I . Properties of the Hydrated Electron at 25° C. 1$ IV. I n i t i a l Hydrated Electron Y i e l d s . 25 V. Calorimetry Data. 52 VI. Calculated Gammas for Various Electron Energies. 68 VII. I n i t i a l Hydrated Electron Yields f o r Various 82 Solutions as Determined by CRS Measurements. VI I I . Cerenkov Intensity i n Solutions of Similar 88 Lifetime of e~ v but with Solutes of D i f f e r i n g Presolvation Scavenging A b i l i t y . IX. I n i t i a l Hydrated Electron Yields as Calculated 93 Using Rate Constants f o r Dilute and Concentrated Scavengers. X. The Reaction of Solvated Electrons with C h i r a l 128 Molecules. ACKNOWLEDGEMENTS I sin c e r e l y thank Dr. David C. Walker f o r h i s guidance, patience and understanding during the course of t h i s work. I would also l i k e to acknowledge the craftsmanship and tec h n i c a l help which I received from both the Mechanical and E l e c t r i c a l Workshops. -1-Chapter I Introduction When i o n i z i n g r a d i a t i o n (electromagnetic or p a r t i c l e ) traverses a medium, energy i s transferred to the medium thus p r e c i p i t a t i n g a succession of phy s i c a l and chemical events. I f the medium i s a polar l i q u i d solvated electrons are formed. The i n i t i a l y i e l d of the solvated electron i n water (the hydrated electron) and the question of whether solvated electrons can possess c h i r a l i t y are the subjects of t h i s t h e s i s . The formation of the hydrated electron from charged p a r t i c l e r a d i a t i o n , what i t i s , and some of i t s more important properties are f i r s t discussed b r i e f l y . - 2 -A. The I n t e r a c t i o n o f High Energy E l e c t r o n s With Matter^" A moving charged p a r t i c l e such as a f a s t e l e c t r o n t r a v e r s i n g a condensed medium l o s e s energy through two fundamental mechanisms: i ) e m i s s i o n o f e l e c t r o m a g n e t i c energy and i i ) c o l l i s i o n a l i n t e r a c t i o n s . The r e l a t i v e importance o f these p r o c e s s e s i s dependent upon the energy o f the i n c i d e n t e l e c t r o n s . E l e c t r o n s w i t h v e r y h i g h energy, E>10MeV, f a v o r r a d i a t i o n e m i s s i o n , w h i l e low energy e l e c -t r o n s p r e f e r i n e l a s t i c c o l l i s i o n s as the mechanism f o r energy l o s s . Loss o f energy through r a d i a t i o n f a l l s i n t o two c a t -e g o r i e s , Bremsstrahlung e m i s s i o n and Cerenkov R a d i a t i o n . Bremsstrahlung i s produced when a charged p a r t i c l e i s de-c e l e r a t e d as a r e s u l t o f p a s s i n g c l o s e t o an atomic nuc-l e u s . E l e c t r o m a g n e t i c r a d i a t i o n i s produced a t a r a t e p r o p o r t i o n a l t o z Z /m where z i s t h e charge on t h e p a r -t i c l e , Z i s the charge on the n u c l e u s and m i s the mass o f the p a r t i c l e . Cerenkov R a d i a t i o n , w h i l e o f i n t e r e s t t h e o r e t i c a l l y and e x p e r i m e n t a l l y as a n a l y z i n g l i g h t , i s unimportant as an energy l o s s mechanism, a c c o u n t i n g f o r < 1% o f the t o t a l energy d i s s i p a t i o n . I t s p r o d u c t i o n i s d i s c u s s e d more f u l l y l a t e r ( S e c t i o n I I . 2 ) . C o l l i s i o n a l i n t e r a c t i o n s can be c l a s s i f i e d as e l a s t i c o r i n e l a s t i c . E l a s t i c s c a t t e r i n g r e s u l t s i n a change o f d i r e c t i o n o f the i n c i d e n t p a r t i c l e without any change i n -3-the p a r t i c l e ' s k i n e t i c energy. An i n e l a s t i c c o l l i s i o n r e f e r s to energy transfer primarily by Coulombic i n t e r a c t i o n of the r a d i a t i o n p a r t i c l e with the electrons of the stopping material. The inte r a c -t i o n produces both excitations and io n i z a t i o n s of the medium. The rate of energy loss, r e f e r r e d to as LET(Linear Energy Transfer) i s represented by the Bethe equation: dx ni 0v^ In m. v 2 E . - (2 V I -fi2 -1 +/? 2)ln2 2I 2 ( 1 -p2) + 1 -/32 + 1/8(1 - V 1 - f i 2 ) 2 ergs/cm ( I - l ) where I = the mean e x c i t a t i o n p o t e n t i a l of the medium N = the number of atoms per cnr e = the electron charge m = the electron mass o v = v e l o c i t y of the electron E = the k i n e t i c energy of the electron i n ergs / 3 = v/c where c i s the v e l o c i t y of l i g h t The r a t i o of the loss of energy by Bremsstrahlung to that of c o l l i s i o n i s approximated by (-dE/dx) J EZ rad ~ (1-2) (-dE/dx) c o l 800 where E i s i n MeV and Z i s the atomic number of the ab-sorber. For 0.5 MeV electrons traversing water c o l l i s i o n a l losses account for >99% of the energy t r a n s f e r . Radiation energy i s not deposited homogeneously into -li-the medium. As the electron slows down i t creates a path, or track, of excited and ionized molecules. Secondary electrons produced from i n i t i a l i o n i zations may have s u f f i c i e n t energy to cause further i o n i z a t i o n s and ex c i t a t i o n s . I f the energy of the secondary electrons is^lOOeV, these subsequent events w i l l occur within a small distance (~10A) from the i n i t i a l event thus forming a clus t e r ( a spur) of ionized and excited species. An averge spur contains 2-3 ion p airs and corresponds to ~100 eV energy l o s s . branch track o O M A I N T R A C K O O O o o o o o o o isolated spurs - 6 0 % o blobs -12% Figure 1 A P i c t o r i a l Representation of the S p a t i a l D i s t r i b u t i o n of Ionizations and Excitations i n Water Following Passage of a.0.5MeV Electron. (Adapted from Reference l c , p. 18) If the secondary electron i s energetic (E> 5000eV), i t i s ca l l e d a S -ray, and may escape the coulombic f i e l d of i t s concommitant .positive ion and create i t s own track. The intermediate case with the secondary electron having an energy of 1 0 0 - 5 0 0 eV creates a so-called blob (a pear-shaped large spur). The inhomogeneity of the energy dep-o s i t i o n i s p i c t o r i a l l y presented i n Figure 1 . For the case of 0 . 5 MeV electrons, 6 0 % of the energy i s deposited i n spurs, 28% i n short tracks and <~ 12% i n blobs. Two c h a r a c t e r i s t i c s , the LET and the dose rate of the r a d i a t i o n , a f f e c t the s p a t i a l inhomogeneity of the energy deposition. As mentioned e a r l i e r the LET i s the rate of energy transfer along each high energy p a r t i c l e ' s track, and as such determines the mean distance between spurs per track. A f a s t electron (E = 0 . 5 MeV) has an average LET of 0 . 0 2 eV/A°, leading to a mean spur separa-t i o n of several thousand Angstroms. In contrast a part-t i c l e s have a high LET ( ~ 2 0 e V / A ° ) and thus create a c y l -i n d r i c a l track of overlapping spurs. The r a d i a t i o n f l u x ( p a r t i c l e s cm s ) d i c t a t e s the mean separation between tracks i n a given time. - 1 - 1 Dose rate ( i n eV g s ) i s used to describe the net average rate of energy deposition. The degree of homoge-neity and the l i k e l i h o o d of i n t e r t r a c k and interspur r e -actions depends upon the dose rate, the l i f e t i m e of the species produced i n the spur and upon t h e i r m o bility. B. The Radiolysis of Water The events subsequent to the deposition of energy - 6 -i n a polar l i q u i d , water i n p a r t i c u l a r , w i l l now be con-3 sidered. These events can be separated into the so-called p h y s i c a l , physico-chemical and chemical stages, depending upon the time-scale and the processes occurring (Table I).^ -The p h y s i c a l stage consists of the tra n s f e r of r a d i -, - 1 5 ant energy to the system i n < 10 sec and includes the io n i z a t i o n and e x c i t a t i o n of the medium (Eq I - l ) by the mechanisms discussed e a r l i e r . H20 V ~ A - _ e - H20+,H20* { I _ 3 ) The physico-chemical stage i n which thermal e q u i l i b -rium i s established follows. The p o s i t i v e ion r a p i d l y -14 reacts with a water molecule (< 10 sec) to form a hyd-roxyl r a d i c a l and a hydronium ion. O H . , (1-4) H 2 0 + + H 20 > H 3 0 + + OH k = 8 x 1 0 1 2 M-!sec-l The r o l e of the excited water molecule i s not understood, but i t may di s s o c i a t e to form hydrogen and hydroxyl rad-i c a l s . H 20* » H + OH ( The fate of the low energy secondary electrons i s complicated. These electrons should be moderated to sub-ex c i t a t i o n l e v e l s ( < 6 . 5 eV) i n 10 ^ s e c , during which time they may t r a v e l up to 30A^. Losing energy to molec-ul a r v i b r a t i o n s the electron i s thermalized (E = 0.025eV) -13 in the order of 10 sec. Thermal electrons have several - 7 -Table I ~ TIME EVENT (Seconds) , -18 10" P a r t i c l e traverses a water molecule 10" ^  E x c i t a t i o n s , Ionizations H 20^~^ H 20 +, e~, H 20 -14 + + 10 Ion-molecule reations H 20 + H 20 — » H ^ O + OH Subexcitation electrons e" =» e^ub 10" 1 3 Molecular v i b r a t i o n s and HO*—> H + OH di s s o c i a t i o n s ^ -13 5 x 10 Electron thermalized e , e*. sub th -12 -10 Electron solvated e., > e _ th aq 10"^"^ D i e l e c t r i c r e l a x a t i o n 1 0 " ^ Radical-radical reactions i n spur; Minimum time f o r d i f f u s i o n controlled reactions i n bulk -9 10 Fluorescence -8 10 Interspur reactions commence _7 10 Homogeneous k i n e t i c behavior 10^ Chemical reactions complete Approximate Time Scale of Events i n Liquid Water Following Passage of a 1 MeV Electron possible fates; they may become solvated (1-6) or they may undergo geminate recombination (1-7) ea-q t 1 - 6 ) e- + H 2 0 + — > H + OH (1-7) 5 or react as "dry" electrons (1-8). e" + S S" Thus at the conclusion of the physicochemical stage + there i s a c o l l e c t i o n of primary species, H, OH, H^ O and e~q, c o l l e c t e d i n a r e l a t i v e l y small volume element. Two models have been proposed i n an attempt to describe the d e t a i l s of the s p a t i a l d i s t r i b u t i o n of these species. 6,7 According to the Samuel-Magee Model, the i n i t i a l species are considered to be randomly mixed with a gaussian d i s -t r i b u t i o n about the spur center because each electron d i d not escape the coulombic f i e l d of i t s p o s i t i v e ion. On the other hand the Lea-Gray and Platzman models allow the electron to escape immediate geminate recombination and to d i f f u s e outward about 50 or 150 A^ at which point solva t i o n occurs. (This distance i s now thought to be unreasonably large.) The H^0 + and OH species thus have a narrower d i s t r i b u t i o n than e" . The physicochemical stage ends with the r a d i o l y s i s products, e ^ , OH and H^ O , s t i l l clustered together i n spurs. During the chemical stage the r a d i c a l s and ions d i f f u s e outward. They may undergo recombination reactions within the spurs (Equations 9-16, Table 2) or a l t e r n a t e l y escape -9-Table II Rate constants for the Reaction of Primary Species i n the r a d i o l y s i s of water Reaction k(M" 1sec" 1) < ea*q + eaq~^ H 2 + 2 0 H " e-q+OH->OHaq e a q + H - ^ H 2 + 0 H a q e a q + H 2 ° 2 ~* 0 H + 0 H a q e +H_0 -^H+H 0 aq 3 2 H+H—»H, H+OH-»H 0 2 OH+OH—*H 0 o 2 2 eaq + H 2 0 H+OH" H+OH — » e aq aq OH+H —> H+H 0 2 2 OH+H202-^ H 20+H0 2 5.5 X 1 0 9 .10 3 .0 X 10 2.5 X 10 10 1.23 X 10 10 10 2 .3 X 10 1.0 X 10 10 10 1.2 X 1 0 Q Acid 1.0 X 1 0 v pH7 5.0 X 1 0 9 < 16 2.7 X 10 ' 6 n o ' , p H 7 1.6 X 10° p H 1 3 4 .5 X 1 0 7 (1-9) (1-10) (1-11) (1-12) (1-13) (1-14) (1-15) (1-16) (1-17) (1-18) (1-19) (1-20) a) Rate constants from Reference 10a. -10-into the bulk of the solution where reaction with solutes or impurities may occur. The i n i t i a l r a d i c a l "concentra-t i o n " i n the spur i s quite large,(perhaps 6 species i n o a spur of 20A radius i . e . ~ 0.3 M), and the intraspur r e --11 -9 actions occur l a r g e l y over the time scale of 10 to 10 sec. The so-called "molecular products", H_ and H O , are formed at t h i s time. Ultimately, i n neutral water, the l i f e t i m e of the hydrated electron i s determined p r i n -c i p a l l y by reaction with the hydrogen ions present, as the hydrated electron i s not very reactive toward water i t s e l f ( k = 16 M"1 s e c - 1 ) 9 . Chemical equilibrium w i l l be obt--3 ained within 10 -1 sec, depending upon the dose-rate and the l e v e l of impurities. C. The Hydrated Electron As was shown e a r l i e r , the hydrated electron, a mem-ber of the family of solvated electrons, i s one of the primary reactive species formed during the r a d i o l y s i s of water. This i n t e r e s t i n g species was f i r s t i d e n t i f i e d i n the early 1960 fs although i t s existence had been postula-ted a decade e a r l i e r . Since i t s discovery, somewhat de-layed by the apparent success of the free r a d i c a l theory (i n which H and OH are the major reactive species) i n ex-p l a i n i n g the r a d i o l y s i s of water, coupled with i t s short l i f e t i m e (0.23 msec), an enormous e f f o r t has been, and i s being, expended i n the study of the hydrated electron. -11-A simple picture of the hydrated electron i s that of an excess electron which i s caught i n a p o t e n t i a l well formed by the p o l a r i z a t i o n ( e l e c t r o n i c , d i p o l a r and or i e n -t a t i o n a l ) of an undefined number of water molecules (Figure Figure 2 Si m p l i f i e d Diagram of a Hydrated Electron showing an Orientation of Polar Molecules about the Electro n . The d e t a i l s of the trapping are unresolved, although i t i s generally thought that the thermalized electron i s caught by a region of accidental p o l a r i z a t i o n of the medi-um. Once l o c a l i z e d the electron then creates a deeper trap through further p o l a r i z a t i o n of the molecules. Var-ious parameters are thought to be important i n t h i s proc-ess including the d i e l e c t r i c relaxation.time of the med-11 12 ium, the s t a t i c d i e l e c t r i c constant, and the structure 12.13 of the solvating medium. ' In the l a s t instance, l i q -uids are known to contain "holes? An electron may become trapped by e l e c t r o s t a t i c forces upon entering such a cav-i t y at which point p o l a r i z a t i o n w i l l further bind the -12-e l e c t r o n Once i t has acquired i t s solvation sheath the hydra-ted electron assumes the nature of a true chemical reac-tant. The various properties i t exhibits are character-i s t i c of solvated electrons i n general. Foremost among these properties i s a broad, intense asymmetric absorption band; A max = 7 1 5 nm ( 1 . 7 3 eV), £ max = I . 8 5 X 1 0 - 1 - 1 1 4 M cm . This much investigated absorption i s generally thought to a r i s e from a 2 p — I s t r a n s i t i o n , although t h i s i s by no means c l e a r . ^ In any case the absorption has been widely used i n hydrated electron studies, allowing low concentrations o f these species to be detected. The band broadens somewhat and the band maximum s h i f t s to s l i g h t -l y lower energies when the temperature i s increased (dE/dT *= - 0 . 0 0 2 9 eV/deg^). Increased pressure causes a s h i f t to higher energies and i s interpreted as decreasing the cav-i t y s i z e * 7 18 The hydrated electron exhibits an EPR spectrum con-s i s t i n g of a sharp s i n g l e t ( l i n e width < 0 . 5 8 a u s s ) with a g-factor close to that of a free electron ( 2 . 0 0 0 2 t . 0 0 0 1 vs. 2 . 0 0 0 2 3 ) . The lack of hyperfine s p l i t t i n g excludes even weak i n t e r a c t i o n of the electron with s p e c i f i c hyd-rogen atoms. The hydrated electron i s a very l a b i l e species with a mobility of = 1 9 8 X 1 0 - 3 C m 2 V " 1 s e c ~ 1 from which a d i f -fusion constant of 4 . 9 5xlO"^cm2sec~^ can be calculated. Only H + and OH" ions have a larger d i f f u s i o n constant than e~q i n water, i n d i c a t i n g that the electron may tunnel or - 1 3 -jump between preexisting traps rather than dragging a s o l -vation s h e l l as i t d i f f u s e s . The hydrated electron i s one of the most elementary reactive chemical e n t i t i e s . It i s a powerful reducing agent (E°= -2.77V), stronger than the hydrogen atom ( E°=.-2.1V) with which i t forms an acid-base p a i r . e - q f H ^ K I-(13) H + GH"—*e- q + H 2 0 I - U 8 ) The a c t i v a t i o n energy f o r most reactions i s small, 3-5 kcal/mole, and often corresponds only to the energy of s e l f - d i f f u s i o n i n water. From studies of d i f f u s i o n con-t r o l l e d " reactions and various t h e o r e t i c a l models a radius of 2.5-3»OA° f o r the hydrated electron has been determined. A l l reactions of the hydrated electron are simply viewed as electron transfer reactions to a substrate containing a low l y i n g molecular o r b i t a l and may be c l a s s i f i e d i n i t -i a l l y as i ) addition to ions (simple reduction) e - + AB n + ^ A B ( n - 1 ) + e- + AB n~ A 3 ( N + 1 ) -aq i i ) addition to neutral molecules ea"q + AB — > AB-where AB~ may undergo further reactions AB" + S ^ AB + S" AB"* + H2Q * ABH + OH" AB" + H + > ABH - H -i i i ) d i s s o c i a t i v e addition e" + AB > A +• B~ aq Before leaving t h i s necessarily b r i e f discussion of the hydrated electron, i t should be mentioned that the formation of e_„ i s by no means l i m i t e d to the action of r a d i a t i o n upon water. Rather the species may be formed i n a number of ways including electrochemically, photo-chemically, and by spontaneous chemical generation. Thus much of the study of the hydrated electron i s generally applicable to a number of d i s c i p l i n e s . Table I I I sumnarizes several physical properties of the hydrated electron. Table III Properties of e" at 25 C aq Optical Absorption Band Max E*max £ (715 nm) d hv/dT (0-100°; O s c i l l a t o r strength ESR g-factor ESR l i n e width Charge Radius of charge d i s t r i b u t i o n ^1/2 ( n e u t r a l D i f f u s i o n Constant Equivalent Conductivity M o b i l i t y AG hyd. AS hyd. AH hyd. o , + E e" + H -aq 1/2H2) 715 nm 1.73 eV 1.85 X 10^  M-1 cm 1 -2.9 X10"3eV/deg 0.71 2.0002 <0.5 gauss -1 2.5-3.0 A" 0.23 msec 4.95 X 10" 5cm 2sec" 1 190 mho cm2 1.98 X 10~3 cm 2V" 1se -37.4 kcal/mole -1.9 cal/mole deg. -38.1 kcal/mole -2.77V Chapter I I The P r i m a r y R a d i o l y t i c Y i e l d o f H y d r a t e d E l e c t r o n s A. G e n e r a l I n t r o d u c t i o n 1 . Y i e l d s I t i s now n e c e s s a r y t o c o n s i d e r t h e q u e s t i o n o f r a d i o l y t i c y i e l d s . A r a d i o l y t i c y i e l d i s r e f e r r e d t o as a G-value — t h e number o f mo l e c u l e s o r i o n s formed o r l o s t p e r 1 0 0 eV o f a b s o r b e d energy. U n t i l r e c e n t l y y i e l d s have g e n e r a l l y been measured under c o n d i t i o n s o f low absorbed dose and low s c a v e n g e r concen-t r a t i o n . The scavengers r e a c t w i t h the p r i m a r y r a d i o l y s i s p r o d & c t s t e form r e a d i l y measurable p r o d u c t s . I n t h i s manner t h e y i e l d s o f the p r i m a r y r a d i o l y s i s p r o d u c t s a r e deduced. Because o f the low s c a v e n g e r c o n c e n t r a t i o n t h e y i e l d s measured r e p r e s e n t t h e number o f p r i m a r y s p e c i e s e s c a p i n g t h e s p u r , and a r e sometimes r e f e r r e d t o as t h e f r e e i o n y i e l d . I n t h e r a d i o l y s i s o f w a t e r e ~ q , H + and O H a r e c o n s i d e r e d t o be t h e major r e a c t i v e p r i m a r y s p e c i e s , and t o g e t h e r c o n s t i t u t e t h e s o - c a l l e d f r e e r a d i c a l y i e l d . H 2 and H 2 0 2 form t h e second group o f p r o d u c t s and c o n s t i t u t e t h e m o l e c u l a r y i e l d . These p r o d u c t s a r e undo u b t e d l y formed by t h e i n t r a s p u r r e a c t i o n s d e p i c t e d i n E q u a t i o n s 1 - 9 , 1 - 1 0 , and 1 - 1 4 , a l t h o u g h e + e" (1-9) aq aq e~ aq + H * H 2 ( 1 - 1 0 ) H + H •> H 2 ( 1 - 1 4 ) -17-there i s also the p o s s i b i l i t y of other sources. Recent 19 work" t e s t i n g the f r e e - r a d i c a l model of water r a d i o l y s i s has demonstrated the importance of the hydrated electron i n the formation of H^. The species surviving microsec-onds a f t e r the absorption of r a d i a t i o n and the y i e l d s of these products are reasonably well established. The over-a l l equation can be written as 4.2H2ewv-*,2.7e" , 2.7H 30 +, 2.70H, 0.6H., 0.45H2, 0.7H 20 2 ( I I_ ^ Understandably these y i e l d s must necessarily show a pH dependence because of the reactions: e" + H,0+ >H?0 + H (1-13) aq J OH" + H — * e" (1-18) aq At low pH the hydrated electrons are converted to hydrogen atoms, while at high pH the hydrogen atoms are converted to hydrated electrons. These reactions occur r a p i d l y i n concentrated solutions, changing the y i e l d s measured over a longer time period. With the now general agreement on the microsecond y i e l d s a t t e n t i o n has s h i f t e d to very early events. What happens between the i n i t i a l event (Equation 1-3) and the s i t u a t i o n represented by the equation (II- 1) H 0*w—> e ~ , H Ot H 20* (1-3) "2 giv i n g the ^ c-sec y i e l d s ? The question of the i n i t i a l y i e l d s , r e f e r r e d to as G° (e~q) f o r the hydrated electron, i s unsettled - i . e . -18-how many hydrated electrons are formed i n the spur before intraspur reactions and solute scavenging can occur; What i s the i n i t i a l y i e l d of the hydrated electron? The question has been approached both t h e o r e t i c a l l y and experimentally. Several models for the r a d i o l y s i s of water have been proposed, and the i n i t i a l y i e l d s of the hydrated electron calculated f o r these models. Perhaps the best known and most widely accepted i s the spur-diffusion model i n i t i a l l y 6a proposed and discussed by Samuel and Magee, and developed 20 21 further by Kupperman and Schwarz. The basis of t h i s the-ory i s the e~ , H, H^O"**, OH and H 2 are a l l contained i n the spur 10 to 10~*^seconds a f t e r thermalization and s o l -vation. They have a s p e c i f i c s p a t i a l d i s t r i b u t i o n which i s assumed to be Gaussian, with the hydrated electrons being on the average 23A° from the center of the spur. These species d i f f u s e outward maintaining t h e i r d i s t r i b -ution (the prescribed d i f f u s i o n assumption). A certain f r a c t i o n escape into the bulk of the so l u t i o n ; the others react. The usee y i e l d s are adopted and the measured rate constants f o r the reactions taking place i n the spur are used to calculate an i n i t i a l y i e l d of hydrated electrons-21 G (e~q) = 4.78. This model predicts a time dependent electron y i e l d as well as e f f e c t i v e time dependent rate constants f o r the hydrated electron. 22 A second model has been proposed by Freeman, This model was o r i g i n a l l y developed f o r hydrocarbons and then exten-ded to polar l i q u i d s . As with the spur d i f f u s i o n model -19-the reactive species are considered a f t e r thermalization. The hydrated electrons are assumed to have a p a r t i c u l a r d i s t r i b u t i o n about H^O"*" and to either undergo geminate n e u t r a l i z a t i o n within the spur or to e s c a p e from the spur. An i n i t i a l electron y i e l d of 4.0 was calculated from N 20 scavenging studies of a H 20/lO%EtOH system. A t h i r d model, proposed by Hamill, considers the fate + _ of H 20 (or H^ O ) and e before t h e i r s o l v a t i o n . It i s postulated that the "dry" correlated charge p a i r H 20 + and e both migrate and can be trapped before s o l v a t i o n . Thus solvation occurs i n competition with n e u t r a l i z a t i o n . S o l -vation i s preferred at greater distances r e s u l t i n g i n a homogeneous d i s t r i b u t i o n of hydrated electrons. Accordingly time-dependent rate constants or y i e l d s should not be ob-served, i n contrast to the spur-diffusion and Freeman mod-e l s . An i n i t i a l y i e l d of hydrated electrons of 3.9 i s calculated from scavenging studies of both the hole and 2 4 the electron. Hamill*s model allows f o r what i s now termed presolvation scavenging. This concept has been the subject of much recent controversy, and i s discussed more com-p l e t e l y i n section II.D as i t has d i r e c t bearing on the experimental r e s u l t s presented l a t e r . In p r i n c i p l e the question of i n i t i a l y i e l d s and the nature of the very early events leading to the Msecond s i t u a t i o n could be simply solved by d i r e c t measurement of the y i e l d , however measuring anything on the picosecond -20-time scale i s fraught with d i f f i c u l t i e s . The experimen-t a l approaches can be divided into two c a t e g o r i e s — d i r e c t and i n d i r e c t . 25 Wolff, et a l . i n 1969 reported the f i r s t d i r e c t observation of the hydrated electron i n the picosecond time scale. The experiments were carried out i n concen-trated scavenger solutions i n which the l i f e t i m e of the hydrated electron was- les s than 300 psec. S i g n i f i c a n t f i n d i n g s were that the hydrated electron was formed i n l e s s 26 than 6 - 8 psec, rate constants ( e ~ q + S) at these high concentrations d i f f e r e d from the corresponding d i l u t e rate 27 + constants, and that solutes other than H decreased the 28 i n i t i a l y i e l d of the hydrated electron. Dosimetry prob-lems prevented an immediate determination of G° + 29,30 4 but l a t e r work reported a value of 4.0-0.2. This i n i t i a l y i e l d i s i n d i r e c t contradiction with that predicted by the widely accepted s p u r - d i f f u s i o n model. Furthermore no time dependent rate constants were observed. The observed decrease i n hydrated electron y i e l d with increasing s o l -ute concentration (other than H +) seems to support Hamill's concept of dry electron scavenging. (The hydrogen ion i s 31 suggested to be unreactive towards the dry electron, ) The decrease in the i n i t i a l hydrated electron y i e l d did not correlate with the r e a c t i v i t y of the solute towards the hydrated electron. The v a r i a t i o n of the rate constants with scavenger concentration i s presumed to be an i o n i c atmosphere e f f e c t . While the complete ramifications of t h i s work are unclear at the moment, i t has c e r t a i n l y cast - 2 1 -doubt on the spur-diffusion model. Recently the measurement of G°(e_ ') has been c a r r i e d a q 2 9 , 3 2 out over the 2 0 0 - 1 0 0 0 psec time scale by Jonah, et a l . Their work supports the value of 4 . 0 obtained by Wolff, et a l . Other workers have attempted to elucidate the nature of the early events i n the r a d i o l y s i s of water, including o G ( e~ q)» through i n d i r e c t experimental means. The time dependence of the i n i t i a l electron y i e l d has been the sub-j e c t of several i n v e s t i g a t i o n s — w i t h c o n f l i c t i n g r e s u l t s . 33 Hunt and Thomas i n 1 9 6 7 reported no s i g n i f i c a n t decay of the hydrated electron a f t e r 0 . 5 nsec, the implication being that the spur reactions are complete at that time. On the other hand Thomas and Bensasson reported an i n i t i a l r apid decay over the f i r s t 7 0 nsec a f t e r a 3 n s e c electron pulse. About 1 5 % of the hydrated electrons were removed i n t h i s time i n t e r v a l , the loss being a t t r i b u t e d to intraspur reactions. The decrease i n e~ reported by Thomas and Bensasson agrees with that predicted by the spur d i f f u s i o n model. Hamill's model which postulates a homogeneous electron concentration would not predict such a rapid decay, how-ever h i s model was subsequently modified to allow f o r s o l -2A vation within the escape distance of the po s i t i v e ion. In t h i s way a rapid i n i t i a l decay can be tol e r a t e d . While a time-dependent y i e l d was thus observed, an absolute - 2 2 -measurement of the i n i t i a l y i e l d was not possible. 35 Other workers have approached the problem by lowering the temperature of a 10M OH s o l u t i o n i n order to slow down the early events to the e a s i l y accessible usee time domain. An i n i t i a l y i e l d of G°(e •) equal to 5.0 was ob-served with approximately one-third of these electrons removed i n the spur. This work lent f i r m support to the spur-diffusion model. Another i n d i r e c t experimental approach involved addition of solutes which precluded various spur reactions that remove e \^ The l i f e t i m e of the hydrated electron was aq extended by t h i s method. Hydroxide ion was added i n suf-f i c i e n t concentration to remove the hydronium ion before i t could react with the hydrated electron. Methanol was added to scavenge both H and OH. Iodide and chloride ions were added to scavenge the hole (H^O^r H-0"*"^  ) as predicted 37 by Hamill. The electron y i e l d was measured at 7.5 nsec, 45 nsec and 120 nsec a f t e r a 5 nsec electron pulse. The y i e l d s were normalized to 2.7 at low solute concentration at 120 nsec. Observations included an i n i t i a l rapid decay of 17% over 50 nsec; no increase i n the hydrated electron y i e l d upon the addition of I~ and C l " ; and an increase i n the y i e l d at 7«5 nsec upon addition of both 0H~ and methanol, the e f f e c t being a d d i t i v e . The absolute electron y i e l d measured with 0H~ and methanol as additives was 5»05i 0.26. A high G(eaq) i s observed which then decreases to -23-the Msec y i e l d . The lack of e f f e c t on the electron y i e l d seen upon the addition of I would seem to preclude scaveng-ing of the p o s i t i v e hole and the concomitant increase i n the y i e l d of hydrated electrons as predicted by Hamill's " model. Thus i t appears that the i n d i r e c t observations of the i n i t i a l y i e l d of hydrated electrons as i n Buxton's work agree n i c e l y with the predictions of the spur-diffusion model. However, the d i r e c t measurements of the picosecond work do not substantiate t h i s model because ( i ) a s i g n i f -i c a n t l y smaller ^ ( e ^ ) i s observed and ( i i ) the predicted change with time was not observed. Direct comparison of the two approaches i s further complicated by two sets of very d i f f e r e n t experimental conditions. The picosecond work of Wolff, et a l . was done under very a c i d i c conditions i n order to keep the l i f e t i m e of the electron very short. On the other hand the i n d i r e c t studies were performed under very basic conditions so as to prolong the l i f e t i m e of the hydrated electron. The question n a t u r a l l y a r i s e s as to whether the early events are the same under these widely disparate conditions. I t has been suggested 3^ that i n very basic solutions an excited water molecule might form a hydrated electron upon d i s s o c i a t i o n (Eq. H-2). 0 H % H 2 0 * - ^ e " + OH + H 20 ( H - 2 ) Such a reaction might r e c o n c i l e the r e s u l t s from the two -24-d i f f e r e n t approaches. However, the very recent work of 32 Jonah, et a l . has measured an i n i t i a l y i e l d of 4.0 at 200 psec under basic conditions and therefore a simple pH e f f e c t would not appear to resolve the discrepancy. The various i n i t i a l hydrated electrons y i e l d s , both t h e o r e t i c a l and experimental, are c o l l e c t e d i n Table IV f o r comparison. This work was undertaken i n an attempt to resolve some of these discrepancies. The only other technique reported to be capable of d i r e c t observation of the i n i t i a l electron y i e l d i s Cerenkov Reabsorption Spectroscopy?^' E a r l i e r work using t h i s technique measured an i n i t i a l hyd-rated electron y i e l d of 3.2^.8 under a c i d i c conditions.*** Later refinements of both the t h e o r e t i c a l treatment and the experimental apparatus offered an apparently simpler and more d i r e c t method f o r attacking the problem. The method of CRS (Cerenkov Reabsorption Spectroscopy) i s , furthermore, not l i m i t e d to either highly a c i d i c or highly basic conditions. Not only can an i n i t i a l G-value be measured, but any pH dependence of t h i s y i e l d could be detected. The method of CRS w i l l be b r i e f l y outlined. The f u l l development can be found i n reference 41. Table IV Theoretical Calculations a) Spur-diffusion b) Freeman c) Hamill Experimental determinations Di r e c t a) 20 psec b) 200-1000 psec c) CRS Indirect a) Low temperature b) Added solutes to preclude spur reactions (Reference 21) 4.0 (Reference 23) 3.9 (Reference 24) G Conditions o 4.0 (Reference 29) a c i d i c 4.1 (Reference 32) a c i d i c & basic 3.2 (Reference 41) a c i d i c 5.0 (Reference 35) basic (lOMNaOH) 5.0 (Reference 36) basic (lMNaOH) I n i t i a l Hydrated Electron Y i e l d s - 2 6 -2. The Method of Cerenkov Reabsorption Spectroscopy When a charged p a r t i c l e traverses a medium at a v e l o c i t y greater than the phase v e l o c i t y of l i g h t i n that medium, energy i s l o s t i n the form of electromagnetic r a d i a t i o n . This emission i s a consequence of the relaxa t i o n of the electron-induced p o l a r i z a t i o n of molecules. The l i g h t , which i s c a l l e d Cerenkov Radiation, i s emitted at a s p e c i f i c angle to the track of the charged p a r t i c l e as given by -1 W ( I I - 3 ) @ — cos x 1/^4 where r\ i s the r e f r a c t i v e index of the medium and / 3 = v/c, the r a t i o of the v e l o c i t y of the p a r t i c l e to the speed of l i g h t i n a vacuum. The number of photons of Cerenkov l i g h t emitted per unit length within a narrow wavelength range 42 i s given by dN./(d-?cl-X)«* t 1 1 " * ) Consequently, as Cerenkov emission i s greater at lower wavelengths, the l i g h t appears blue to the eye. While Cerenkov r a d i a t i o n i s not a s i g n i f i c a n t pathway f o r energy loss by an electron beam, i t has been used as an analyzing l i g h t i n a number of cases. I t s advantages are that the duration of the l i g h t pulse i s synchronized with the electron beam. The extremely short duration of electron pulses thus affords access to the picosecond time scale. -27-In the case of water the Cerenkov emission cuts-off at an electron energy of about 260 KeV. This means that the Cerenkov emission i n water from the electron source used i n t h i s work i s emitted i n the f i r s t few tenths of a mm and predominately i n the same d i r e c t i o n as the electron beam. The actual depth d i s t r i b u t i o n i n aluminum was measured (Section II-B) and i s graphed i n Figure 1 4 . An experimental procedure f o r using the Cerenkov l i g h t as an analyzing l i g h t source i n conjunction with a Febetron electron source has been developed previously; ' ' B a s i c a l l y , the Cerenkov l i g h t pulse produced by the electron beam i s p a r t i a l l y reabsorbed by the r a d i a t i o n species also produced by the electron beam. The experimental observables are the unabsorbed Cerenkov l i g h t I Q and the p a r t i a l l y reabsorbed Cerenkov I. I Q i s obtained i n an experiment i n which a l l absorbing species are eliminated by the ad-d i t i o n of suitable concentrations of reactive solutes. The amount of absorption depends upon such fac t o r s as the r a d i o l y t i c y i e l d of the absorbing species, i t s e x t i n c t i o n coefficient and i t s l i f e t i m e r e l a t i v e to the Cerenkov pulse. Unfortunately, the treatment i s not a straightforward ap-p l i c a t i o n of Beer's Law because both the l i g h t source and the concentration of the absorbing species have a complicated time and s p a t i a l dependence a r i s i n g from the time p r o f i l e and energy deposition of the electron beam. The r e l a t i o n s h i p between the Cerenkov l i g h t pulse and the dose deposition p r o f i l e (depicted i n Figure 1 4 ) i s c r i t i c a l to the Cerenkov - 2 8 -reabsorption experiments, and i s dealt with i n d e t a i l i n a l a t e r section. The Cerenkov photons traverse a region of high deposited dose, but any absorbing species formed at the front of the c e l l would be traversed by a smaller f r a c t i o n of the photons than species formed toward the back of the c e l l . A t h e o r e t i c a l treatment of Cerenkov Reabsorption Spec-troscopy has been developed previously*** f o r three d i f f e r e n t cases: ( i ) s h o r t - l i v e d 10^° sec) absorber, ( i i ) long--8 l i v e d (> 10" sec) absorber, and ( i i i ) the general case. The treatments w i l l be b r i e f l y recapitulated here with s p e c i a l attention paid to the f i r s t case. The reader i s refe r r e d to the references f o r the complete treatment. Case I. In t h i s case the concentration of the absorbing species at any point during the pulse i s at a rad i a t i o n - s t a t i o n a r y l e v e l governed by i t s l i f e t i m e and the dose rate (Eq.II-5) dC/dt = 0 = c*GR - k 2 [ S j [ c ] (II-5) The time dependence i s removed by the assumption that the maximum of the dose pulse coincides with both the maximum in the reference:Cerenkov I Q ( t ) and with the maximum i n the absorbed Cerenkov I ( t ) . The maximum or peak i s chosen f o r convenience of handling the data. Also at t h i s point the electron beam i s e s s e n t i a l l y monoenergetic and a l l electrons thus contribute equally to Cerenkov production. -29-Th e problem of the s p a t i a l dependence of the l i g h t source and the absorbing species was solved originally*** by s p e c i f y i n g that only a cert a i n f r a c t i o n $ of the t o t a l dose was a v a i l a b l e f o r absorption, or a l t e r n a t e l y that an e f f e c t i v e pathlength was represented by tf. The cor-r e c t i o n f a c t o r was then estimated from the dose depth and Cerenkov depth d i s t r i b u t i o n s . The former was obtained hk from blue cellophane dosimetry using aluminum spacers, and estimated f o r water. The Cerenkov function was c a l -culated from the Franck-Tamm equation. For the case of 550 KeV electrons (mean peak energy incident upon the l i q -uid) a $ of 0.7i".l was obtained. Beer's Law i s now applied: ebC I /I = exp (H-6) where the product bC may be taken as b C = y J a d C ' ( x ) d x (II-?) 0' where ^ i s a correction f a c t o r applied to obtain an " e f f e c t i v e pathlength" and to thus correct f o r the p a r t i a l overlap of the Cerenkov photons and deposited dose. The l i m i t a^ i s the distance at which the dose deposition be-comes zero. The i n t e g r a l can be r e l a t e d simply to the t o t a l incident r a d i a t i o n f l u x , thus ad J C» (x)dx = R^GT/LxlO 2 (H-8) 0 Rjyj i s the incident r a d i a t i o n f l u x at the peak, T i s the -30-l i f e t i m e of the absorbing species, and L i s Avogadro's number. Combining equations H-6 and II-B gives l o g ( I 0 / l ) = GGT (10R i /L) (II-9) Experimentally, f o r the absorbing species being the hydrated electron, the s h o r t - l i v e d condition i s met by a high scavenger concentration. The reference Cerenkov at the maximum I M and the absorbed Cerenkov I„, are obtained o F i M spectrophotometrically with a detection system s u f f i c i e n t l y f a s t as to be able to properly display these maxima. Case II In the case of a long-lived absorbing species -8 ( r > 10 sec) an approximate method was developed to obtain the spectra. The number of absorbing species at any time i s given by the t o t a l accumulated dose to that point, as decay w i l l be i n s i g n i f i c a n t i n the time scale of the r a -d i a t i o n pulse. I t i s assumed that the Cerenkov l i g h t wave-form and the dose waveform have the same time dependence. Furthermore a correction factor i s applied to obtain the e f f e c t i v e pathlength as i n Case I. The f i n a l expression becomes I /I = G€ (23.03* D/L) 0 P , (11-10) where D p i s the t o t a l r a d i a t i o n per pulse i n eV/cm. Case III Expressions f o r I Q ( x , t ) and c(x,t) were obtained from beam current and dose depth measurements combined with the Bethe stopping power r e l a t i o n s h i p and the Franck-Tamm -31-equation f o r the production of Cerenkov r a d i a t i o n . It was assumed that the absorbing species was a primary r a d i o l y t i c product formed i n < 10"-^ sec. Therefore i t s rate of f o r -mation i s proportional to the dose rate and i t s rate of decay i s proportional to a f i r s t order rate constant k. Furthermore, i t i s assumed that the time and space v a r i a -bles can be integrated separately. S i m i l a r l y the time and space dependence of the Cerenkov l i g h t production are separated. To obtain an e x p l i c i t function f o r the dose, the experimental waveform was approximated by a sine function g(t) = s i n W r p (11-11) The Cerenkov waveform was calculated from the observed, dose waveform coupled with the Franck-Tamm r e l a t i o n s h i p , and was also represented by a sine function. The f i n a l expression derived was (11-12) where 1" = duration of the dose pulse P T = duration of the Cerenkov l i g h t pulse Q-c = the depth at which Cerenkov l i g h t ceases to be produced and the other symbols have t h e i r meanings as previously given. B. Experimental 1. The_ Febetron The electron generator used i n t h i s work was a model 730/2667 Febetron. This p a r t i c u l a r model i s a model 701 which de l i v e r s a 30 nsec pulse modified with a 2667 pulse shortener to produce a 3 nsec pulse. The discharge of a Marx Surge network creates a 6 0 0 kV, 30 nsec pulse which i s then shortened by an inductive pick-up method i n a spark chamber to 3 nsec while maintaining the 600 kV amplitude. The pulse i s then applied to the cathode of a f i e l d emis-sion tube to produce a pulse of electrons. This f i n a l pulse of electrons emerging through a very t h i n anode, while not monoenergetic, has an " e f f e c t i v e " beam energy of 300 KeV. The peak beam current i s 5000 amperes and an energy of approximately 7 joules i s delivered. The out-standing feature of t h i s accelerator i s i t s extremely high 28 -1 -1 dose rate of ^ 10 eVg s The actual electron f l u x as a function of time (the current p r o f i l e ) i s measured using a Faraday Cup (Sec. II.B) 3/2 46 The dose rate i s thought to be proportional to I , and i s thus obtained from the Faraday Cup measurements. Widths at h a l f height of the current pulse are t y p i c a l l y about 2.7 nsec and the dose pulse i s narrower. Two electron tubes, d i f f e r i n g i n energy density, the 5510 and 5515, can be used with the Febetron. The 5515 tube, giving the higher energy density, was used f o r a l l -33-the CRS experiments reported. A comparison of the energy p r o f i l e of the two tubes i s given i n Figure 3. The success or f a i l u r e of CRS experiments depends to a large extent on the r e p r o d u c i b i l i t y of the electron beam so as to obtain values f o r I and I which can be meaning-o f u l l y compared. Unfortunately the Febetron i s not as good i n t h i s respect as would be hoped f o r . The r e p r o d u c i b i l i t y i n several respects i s of i n t e r e s t — t h e t o t a l dose deposited per pulse, the on-axis dose, and the pulse shape. The on-axis dose was measured using a small calorimeter (Section II.B) and had a standard deviation of ± &% as compared with e a r l i e r work givi n g an rms of - 7%* P r i o r work had also shown pulse widths of 2.8 to 3.3 nsec and v a r i a t i o n s 39 i n pulse height of t 7% using a Faraday cup to observe the current p r o f i l e . The measurements performed here (Section II.B) using a more v e r s a t i l e o s c i l l o s c o p e showed an even greater v a r i a t i o n i n pulse heights, the standard deviation being 10%. Much of the v a r i a t i o n resulted d i r e c t l y from irreproducible pulse shapes. Figure 3. Electron Dose vs. Depth P r o f i l e i n Aluminum; £ inch from the tube face. (Fi e l d Emission data, Technical B u l l e t i n , V o l . 7) - 3 5 -2 . Materials and Solutions 1 . Water: Laboratory d i s t i l l e d water was r e d i s t i l l e d from a c i d i c potassium dichromate. This water i s hence-f o r t h referred to as doubly d i s t i l l e d (2D) , and was used i n the preparation of a l l aqueous solutions. 2 . Perchloric Acid Solutions: The p e r c h l o r i c a c i d solutions ( 0 . 5 0 , 0 . 7 5 , 0.90, 1 . 0 0 , 1 . 2 5 , 1 . 5 0 and 2 . 0 0 M ) were prepared from BDH A r i s t a r p e r c h l o r i c a c i d and 2D water. 3. Acetone Solutions: A stock sol u t i o n of acetone (Fisher Reagent) with 2D water was prepared. The subse-quent acetone solutions (0.60, 0 . 8 0 , 1 . 0 0 , 1 . 2 0 , and 1 . 5 0 M) were prepared by d i l u t i o n of the stock s o l u t i o n . A f r e s h l y prepared stock s o l u t i o n of NaOH (Analar, BDH p e l l e t s ) was used to adjust the hydroxide ion concentration to 0 . 0 1 , 0 . 1 0 and 1 . 0 0 M f o r each acetone concentration. Perchloric acid (BDH, A r i s t a r ) was added to obtain a hydrogen ion concentration of 0 . 1 0 M. A l l the solutions were main-tained at an i o n i c strength of IM by addition of sodium perchlorate (Fisher C e r t i f i e d ) . When indicated, methanol (Fisher C e r t i f i e d ) was added. 4 . Nitrate Solutions: Nitrate ion solutions of varying concentration ( 0 . 2 5 to 0 , 6 0 M) were prepared from sodium -36-n i t r a t e (Fisher Reagent) and 2D water. Sodium hydroxide (BDH Analar), sodium perchlorate (Fisher C e r t i f i e d ) , p e r c h l o r i c acid (BDH Analar A r i s t a r ) and methanol (Fisher C e r t i f i e d ) were added as indicated. Radiation C e l l The r a d i a t i o n c e l l used for the CRS experiments i s shown i n Figure 4. The c e l l was f i t t e d to the front of the Febetron, and was f i l l e d from a s l i g h t l y pressurized a l l glass flow system. -37-g IN F i g u r e 4 R a d i o l y s i s C e l l f o r Cerenkov R e a b s o r p t i o n S p e c t r o s c o p y E x p e r i m e n t s a- Aluminum f l a n g e f i t t e d t o F e b e t r o n f a c e b- 0.001" s t a i n l e s s s t e e l e l e c t r o n window c- T e f l o n 0 - r i n g d- S t a i n l e s s s t e e l body o f c e l l w i t h e n t r a n c e and e x i t p o r t s w i t h s t a n d a r d t a p e r f i t t i n g s e- S i l i c a window f - Aluminum cover w i t h r e s t r i c t i n g i r i s g- I s o p r e n e 0 - r i n g -38-3. Experimental Lay-out  Optical Train A schematic diagram of the experimental layout i s shown i n Figure 5. The Cerenkov l i g h t transmitted through the c e l l was collimated by a lens (f=3.75cm) and r e f l e c t e d by a mirror at 45? A second lens on the other side of the lead s h i e l d i n g focused the l i g h t onto a pin photodiode (Hewlett-Packard 5802-4207). Wavelength s e l e c t i o n was accomplished with a continuous interference l i n e f i l t e r (VERIL S200) located between the second lens and the photo-diode. The transmission c h a r a c t e r i s t i c s of the f i l t e r were measured i n the same configuration as i t was used i n the CRS experiments. White l i g h t from a tungsten lamp was collimated and passed through a Bausch and Lomb grating monochromator (1350 groves/mm, 1.34mm entrance s l i t , 1.50mm exi t s l i t ) , and was then passed through the interference l i n e f i l t e r and focused on the photodiode. The c a l i b r a t i o n of the grating monochromator was checked with a mercury lamp (Bausch and LombSP200). The interference l i n e f i l t e r was operated at a s l i t width of 12mm. The transmission of the f i l t e r was recorded on a Hewlett Packard s t r i p chart recorder as the grating monochromator was scanned. A t y p i c a l transmission curve f o r the f i l t e r (Figure 6 ) had a half-width of 15 nm. A se t t i n g of 29 on the control rod indicated a wavelength of 612 nm. No transmission as a r e s u l t of pin holes or burns i n the f i l t e r was -39 -E F G Figure 5 Experimental Set-up f o r Cerenkov Reabsorption Spectroscopy A- Febetron B- Radiolysis c e l l C- Collimating lens D- Mirror E- Lead shielding F- Focusing lens G- Interference l i n e f i l t e r H- Neutral Density f i l t e r s I- Photodiode J - Oscilloscope - 4 0 -+ 575 650 Figure 6 Transmission C h a r a c t e r i s t i c s of the Interference Line F i l t e r (set at 2 9 on control rod) - 41 -observed. Wratten n e u t r a l d e n s i t y f i l t e r s were p l a c e d between the f i l t e r and the photodiode t o m a i n t a i n a r e l -a t i v e l y constant l e v e l o f l i g h t on the d e t e c t o r ( 3 0 - 6 0 mV a c r o s s 5 0 J T ) . E l e c t r i c a l D e t e c t i o n : The Cerenkov l i g h t i n t e n s i t y was monitored using a Hewlett-Packard s i l i c o n pin photodiode ( 4 2 0 7 ) having a s e n s i t i v e area of 8 X 10"^cm and operating i n the ele c -t r o n i c configuration shown i n Figure 7 . The response time i n such a configuration i s quoted as < 1 . 0 nsec. The si g n a l from the photodiode was transmitted through 5 Oil cables (less than 3 feet) into a Tektronix 7 9 0 4 o s c i l l o s c o p e equipped with a 7 A 1 9 v e r t i c a l a m p l i f i e r quoted as having a risetime of 0.8 nsec. The si g n a l was photographed using 1 0 , 0 0 0 ASA Polaroid f i l m . Simultaneous external fogging was used. The camera lens was f/l« 2 and gave a 1 : . 5 reduction f o r a d d i t i o n a l photographic speed. The oscilloscope was triggered externally from a wire so placed that i t would pick up e l e c t r i c a l noise a s s o c i -ated with the Febetron's electron pulse. The signals were generally recorded with oscilloscope settings of 2 nsec/div and 1 0 mV/div. A t y p i c a l trace i s shown i n Figure 8. Measurements: The c e l l was f i l l e d with the solut i o n to be i r -radiated. When the Febetron was f i r e d , the Cerenkov l i g h t which was produced was p a r t i a l l y reabsorbed and transmit-ted to the detector. The oscilloscope s i g n a l was photo-graphed. Solutions were replaced after, alternate shots, although generally no change i n absorption was noticed for several pulses. However small bubbles were v i s i b l e -43-+ 15 V Figure 7 Photodiode C i r c u i t r y H 1 1 ( f U _^ 2:7 nsec^ Figure 8" A T y p i c a l Trace of P a r t i a l l y Reabsorbed Ceren Light. 0.40M NO3 Solution. Horizontal: 2 nsec/div. V e r t i c a l : 10 mV/div. i n the c e l l a f t e r about three p u l s e s . Each experimental p o i n t o b t a i n e d was the average o f 5 to 15 i n d i v i d u a l measurements. - 4 6 -4. Dosimetry In order to measure an absolute G-value by the method outlined previously, the maximum dose rate delivered by the Febetron must be evaluated. This dose rate i s measured i n two parts: ( i ) measurement of the current p r o f i l e ^ o f the Febetron and ( i i ) measurement of the t o t a l dose d e l i v -ered. From the current p r o f i l e , a voltage p r o f i l e i s de-ri v e d over which the t o t a l dose i s d i s t r i b u t e d i n order to obtain the maximum dose rate, ( i ) Faraday cup measurements The current p r o f i l e of the electron pulse from the Febetron (5515 tube) was measured using a Faraday cup (FE Model 1653). A small (0.5 mm) portion of the beam was col l e c t e d by the l o g a r i t h m i c a l l y tapered cup. The si g n a l was transmitted through a 50 ohm system using GR cable, attenuators and connectors into a Tektronix 7904 o s c i l l o -scope equipped with a d i r e c t access 7A21N v e r t i c a l ampli-f i e r (risetime of ^ 0.35 nsec). The s i g n a l was photographed on 10,000 ASA Polaroid f i l m (type 410) using prefogging techniques. A time p r o f i l e of the dose pulse was calculated from the current pulse using the r e l a t i o n s h i p that the dose o / o 46 (or voltage) i s proportional to I J > / . O r i g i n a l l y i t was thought that the electron tube acted as a constant impedance 2 and that the dose would be proportional to I . However, i t i s now believed that the three-halves r e l a t i o n s h i p -47-w i l l give a better approximation of the voltage pulse as i t allows f o r the fact that the electron beam i s not mono-energetic and that the electron energy changes with pen-e t r a t i o n into the stopping material. Rather than constant impedance, the electron tube i s believed to exhibit constant permeance. The time p r o f i l e of the dose pulse normalized at the current maximum i s shown i n Figure 9 f o r a t y p i c a l current p r o f i l e , ( i i ) Calorimetry The t o t a l energy deposited by the electron pulse was determined by the method of adiabatic c a l o r i m e t r y ^ * ^ Using a thermocouple (alumelcalomel, 0.05 mm diameter wires from Omega Engineering Corp. Stanford, Conn.) the temperature increase of a small d i s c of high q u a l i t y aluminum was meas-ured. The calorimeter was placed i n the i r r a d i a t i o n c e l l i n a configuration i d e n t i c a l to that from which the Cerenkov l i g h t was coll e c t e d i n the CRS experiments. The energy deposited by the beam was then calculated using the r e l a -t ionship AV = — (11-13) c m where cr = s p e c i f i c heat of the calorimeter (0.896 joules g" 1^"" 1 f o r aluminum) to « s e n s i t i v i t y of the thermocouple (0.0000400 v ° c ~ 1 ) m = mass of the calorimeter W = t o t a l energy deposited i n joules AV = change i n voltage Figure 9 A t y p i c a l current p r o f i l e as measured with a Faraday cup. Measured current p r o f i l e . Dose p r o f i l e calculated using I and normalized to current maximum. 2 Dose p r o f i l e calculated using I and normalized to current maximum. - 4 9 -Four calorimeters (Figure 1 0 , T a b l e V) of various design and with varying cooling c h a r a c t e r i s t i c s were used to obtain the dose deposited. A l l four f i t t e d into the same metal flange which held the aluminum disc i n the center of the i r r a d i a t i o n c e l l d i r e c t l y behind the s t a i n l e s s s t e e l electron window. The p o s i t i o n of the thermocouple was found to be very important. The aluminum discs were of s u f f i c i e n t thickness (0.70 mm) to stop a l l the electrons. The thermo-couple was connected to an operational a m p l i f i e r ( X 1 0 , X 1 0 0 , X 1 0 0 0 ) . Depending upon the rate of cooling the s i g n a l was recorded on e i t h e r a s t r i p chart 5 mV recorder (Hewlett-Packard Model 6 8 0 , risetime of 0 . 5 sec) or on an o s c i l l o s c o p e . The cooling curves were then extrapolated to zero time. In t h i s manner the change i n voltage created i n the thermo-couple was obtained. As the thermocouple s e n s i t i v i t y , the s p e c i f i c heat of aluminum, the weight of the aluminum d i s c and i t s area were a l l known qua n t i t i e s , the energy deposited per u n i t area by the electron pulse could by calculated. Calorimeter D was the easiest to work with as i t s cooling curve obeyed Newton's Law of Cooling -50-F i g u r e 10 C a l o r i m e t e r s I. Design f o r calorimeters A and B a. Aluminum flange which attaches to Febetron as i n the CRS r a d i o l y s i s c e l l b. 0 . 0 0 1 " s t a i n l e s s s t e e l electron window c. C e l l body designed as i n CRS r a d i o l y s i s c e l l , but with center hole for calorimeter. Another body had the holes positioned at various distances from the center. d. Aluminum disc (3 or 6mm diamter) e. Thermocouple f . Teflon holder I I . Design f o r Calorimeter C Interchangeable with calorimeter D and having the same key. I I I . Design f o r calorimeter D a,b,c(except only center l o c a t i o n ) , d,and e have the same meaning as i n I. f - boron n i t r i d e plug g- annular t e f l o n r i n g with bfass terminals f o r the thermocouple wires h-amplifier i - o s cilloscope IV. Head-on view of boron n i t r i d e housing a- aluminum d i s c b- perforated housing - 5 1 --52-TABLE V Calorimetry Data Calorimeter Ch a r a c t e r i s t i c s Dose Deposited (eV cm"2 X 10" 1 9) Diameter (mm) Weight (mg) Plug A 3.0 12.9 t e f l o n 2 . 9 ± . 3 B 6.0 55.6 t e f l o n 2.5 ± .4 C 4.0 19.2 boron n i t r i d e 3.0 * .3 D 4.0 24.5 boron n i t r i d e 3.0 ± . 2 -53-a f t e r an i n i t i a l overshoot of ~1 sec. Such behavior of s i m i l a r calorimeters has been observed by o t h e r s ^ and the overshoot i s a t t r i b u t e d to non-uniform heating. A t y p i c a l cooling curve and logarithmic plot are shown i n Figure 11 A l i n e a r extrapolation of the l o g plot to zero time was possible. The dose deposited (an average of ten pulses) was 3.0 I 0.2 I 10 eV c m . Neither calorimeter A or B obeyed Newton's Law of Cooling. This behavior i s a consequence of the p o s i t i o n of the thermocouple and the design of the calorimeters. Looking at a t y p i c a l trace (Figure 12), a sharp temperature increase i s observed i n i t i a l l y . However, rather than an overshoot, the voltage change recorded i s too low. The thermocouple junction i s touching the t e f l o n mount as well as the aluminum d i s c , and the recorded ^mV i s a function of the aluminum disc plus the cooler t e f l o n surroundings. The cooling curve recorded i s b a s i c a l l y that of the a l u -minum as the time scale i s too f a s t f o r t e f l o n cooling to be s i g n i f i c a n t . Because the t e f l o n plug acts as a heat sink the cooling of these calorimeters occurs more slowly than i n those allowing free a i r c i r c u l a t i o n . The curves were extrapolated to zero time from the log A V vs. time curve with a smooth curve i n the manner shown i n Figure 12. r-—i . 1 f , , . h o r i z o n t a l : 500msec/div v e r t i c a l : 50 mv/divi (100 X a m p l i f i c a t i o n ) 1 1 1—1 1 1—* r 2.0, T I M E ( S E C ) F i g u r e n C o o l i n g curve and l o g p l o t f o r c a l o r i m e t e r D /(mV readings obtained, with 100 X amplification) -56-Th e deposited dose obtained was A - 2.9 1 0.3 X 10 1 9eV cm"2 B - 2.5 t 0.4 X 10 1 9eV cm"2 The lower value for Calorimeter B indicates a s l i g h t f a l -l i n g - o f f of the dose with distance from the center of the beam. Calorimeter C, with a boron n i t r i d e plug (perforated to allow a i r c i r c u l a t i o n ) and a 4 mm aluminum d i s c , had the thermocouple pinched into the corner. This calorimeter showed an i n i t i a l overshoot (Figure 13) followed by rapid cooling (400-500 msec). (The t e f l o n calorimeters had cooling h a l f - l i v e s of ~ 14 sec.) The cooling curve obeyed Newton's Law although a large f r a c t i o n of the curve was l o s t during the time of the i n i t i a l overshoot. A dose of 3.0 ± 0.3 X IQ -2 10 7eV cm .was calculated. A comparison of the c h a r a c t e r i s t i c s of the various calorimeters and the dose determined i s given i n Table V. -57-C o o l i n g curve and l o g p l o t f o r c a l o r i m e t e r C ' (100 X amplification of.,raV j 3 c a l e ) - 5 8 -From the t o t a l dose deposited and the shape of the dose pulse the maximum dose rate was calculated. The voltage pulse was converted to a rectangular pulse of the same height and t o t a l area. The dose per cm was then divided by the duration (sec) of the rectangular pulse, giving a maximum incident r a d i a t i o n f l u x Rj^ of 1 . 0 ± 0 . 1 x 1 0 2 ^ eV cm"2 sec~^-. This f i g u r e i s about 3 0 % lower than an e a r l i e r determination i n t h i s laboratory. ^ 9 The difference a r i s e s because ( i ) a lower dose deposited was measured and ( i i ) a broader voltage pulse r e s u l t s from using the I ^ / 2 r e l a t i o n s h i p rather than the I 2 . ( i i i ) S p a t i a l Characteristics of the Electron Beam Calorimeters A and B were also used to measure the v a r i a t i o n i n i n t e n s i t y across the diameter of the electron beam. A set of flanges was made which held the calorimeters at distances of 2 , 4 , 6 , 8 , 1 0 , 1 2 , and 1 4 mm from the center of the beam. The dose deposited at each of these positions was measured as described previously. The dose dropped o f f with distance from the center as shown i n Figure 1 4 . mm FROM CENTER mm FROM CENTER Figure 1 4 Dose deposited as a function of distance from the center of the electron beam a) 3mm diarrfjer aluminum disc-calorimeter A b) 6mm diameter aluminum disc-calorimeter B -60-5. Cerenkov Depth and Dose Depth Functions In any treatment of CRS a knowledge of the s p e c i f i c s p a t i a l dependence of both the Cerenkov l i g h t and dose depth functions, L(x) and D(x), re s p e c t i v e l y i s necessary. Once known these functions may be used to calculate a $ to use i n the integrated form of Beer's Law or they may be applied d i r e c t l y to the d i f f e r e n t i a l form of the law. To t h i s end ( i ) , the actual Cerenkov depth p r o f i l e i n aluminum was measured; ( i i ) the dose-depth p r o f i l e f o r 500, 550 and 600 KeV electrons i n aluminum was obtained by i n t e r p o l a t i o n of previously calculated p r o f i l e s f o r 4OO and 700 KeV electrons; and ( i i i ) , a n a l y t i c a l expres-sions were obtained f o r the Cerenkov depth (from i ) and the dose depth (from i i ) p r o f i l e s . An empirical value of Y was calculated by summing^over the whole depth,the f r a c t i o n of the t o t a l of Cerenkov photons produced at the various depths m u l t i p l i e d by the f r a c t i o n of the t o t a l dose traversed. (Equation I I - I 3 ) . ( i ) The Cerenkov depth p r o f i l e was measured using the c e l l pictured i n Figure 15. This c e l l was designed c e l l used f o r the CRS experiments. Thus i t was positioned o a so that i t could be inserted into the larger r a d i o l y s i s -61-Figure 15 C e l l f o r Cerenkov i n t e n s i t y vs. depth measurements 6> V K (b) ( a ) (a) a- flange attaches to Febetron face b-r O-ring c- s t a i n l e s s s t e e l electron window d- O-ring e- c e l l holder (body of CRS c e l l ) f - Cerenkov i n t e n s i t y c e l l (see (b)) g- r e s t r i c t i n g i r i s and c e l l cover (b) a- set of aluminum spacers of various thickness b- O-ring to hold covers i n place c- s t a i n l e s s s t e e l c e l l body d- s i l i c a window -62-d i r e c t l y behind the s t a i n l e s s s t e e l electron window in the exact configuration as i n the CRS experiments and with the i d e n t i c a l o p t i c a l t r a i n . This m i n i - c e l l could accomo-date a v a r i e t y of aluminum (99% pure) spacers ranging from ~ 1 to 30 thousandths of an inch. The c e l l i t s e l f was f i l l e d with a 3M acetone s o l u t i o n to prevent any s i g -n i f i c a n t reabsorption of Cerenkov photons by hydrated electrons. The m i n i - c e l l could be inserted and removed without a l t e r i n g the p o s i t i o n of the Febetron or the op-t i c a l t r a i n . The Cerenkov l i g h t emitted at 612 nm was detected and measured i n the same fashiontas i n the CRS experiments. The Cerenkov depth d i s t r i b u t i o n measured i s shown i n Figure 16. This observed d i s t r i b u t i o n was f i t by two exponential functions using a non-linear l e a s t squares program on f i l e at the UBC computer center (BMDX85). y = A exp(-x 2/B) + C exp(-x 2/D) ( I I -where x i s the depth from the c e l l ' s electron window. Four parameters (A, B, C, D) were varied to give the best f i t i n which A = 79.4, B = 1.63 X 10 4, C = 210.0, and 3 D = 1.24 X 10 . The calculated f u n c t i o n . i s shown i n F i g -ure 16 along with the experimental curve. ( i i ) Dose-Depth D i s t r i b u t i o n The electrons at the pulse peak are e s s e n t i a l l y mono-energetic, t h e i r energy a f t e r passing through the s t a i n l e s s t e e l window being ~550 KeV. The dose depth curve i n - 6 3 -F i g u r e 1 6 Cerenkov l i g h t - d e p t h and Dose-depth c u r v e s E x p e r i m e n t a l Cerenkov-depth c u r v e v...-* F i t t e d Cerenkov-depth curve B: X - -C a l c u l a t e d dose-depth c u r v e f o r 550KeV e l e c t r o n s i n aluminum -.K F i t t e d Dose-depth curve INTENSITY -65-A l f o r 500, 550 and 600 KeV electrons was calculated by i n t e r p o l a t i o n of published data f o r 4 0 0 and 7 0 0 KeV elec-49 trons. The d i s t r i b u t i o n s have been calculated f o r a plane perpendicular electron source allowing f o r both scatter-ing and slowing down. The family of curves are shown i n Figure 17 and were f i t using the BMDX&5 program to the function Aexp((x - x ) 2/B) (11-15) max with A and B being adjustable parameters. The best f i t gave A = 2.60 and B = 7.33 X 10? -67-( i i i ) Calculation of Gamma A Gamma was calculated using the experimental Cerenkov depth d i s t r i b u t i o n and each of the dose-depth d i s t r i b u t i o n s f o r 500, 550, and 600 KeV electrons. The s p a t i a l over-lap of the Cerenkov-depth and dose-depth functions f o r the 550 KeV case i s depicted i n Figure 16. A program was written whereby the f r a c t i o n of Cerenkov photons produced at various depths was weighted by the f r a c t i o n of the dose which these photons traversed. Integration of the exponentials was performed using Simpson's Rule and a computer l i b r a r y program UBC SQUANK. The sum of the v a r i -ous weighted f r a c t i o n s equals Gammas were c a l c u l a -ted i n t h i s manner f o r the various electron energies and are col l e c t e d i n Table VI. The true electron energy i s thought to be 550 KeV and the value of V to be used was therefore taken to be 0.706. Since the energy of the electrons are ce r t a i n l y expected to be i n the range of 0.525 to 0.575 MeV at the pulse peak, the gammas f o r 0.500 and 0.600 MeV electrons evaluated by t h i s method would represent absolute outer l i m i t s . The value f o r t has been calculated in aluminum rather than water because of the experimental d i f f i c u l t i e s which would be faced i n attempting to measure the Cerenkov i n t e n s i t y i n water, and the dose-depth data was ava i l a b l e i n aluminum. -68-Table VI Calculated gammas for various electron energies Energy Electron  (KeV) 500 0.669 550 0.706 600 0.736 700 0.787 -69-Because i t represents the r a t i o of the two depth p r o f i l e s , the gamma calculated for aluminum should be a reasonable representation of the actual factor i n water. With these l i m i t a t i o n s i n mind, gamma i s quoted as 0.71 - 0.03. -70-C. T r e a t m e n t o f t h e D a t a The Cerenkov l i g h t transmitted through the solutions was measured as described e a r l i e r . The concentration of the electron scavenger i n each solution was such that the data could be treated by the approximation f o r a short-l i v e d absorber - i . e . the hydrated electron had a l i f e -time of < 1 0 ^ s e c . Under these conditions l o g ( l / l 0 ) M = Ger(10R//L) (II - S ) where the symbols have t h e i r e a r l i e r meaning. The equation i s the integrated form of Beer's Law with the concentration of the hydrated electron at a r a d i a t i o n stationary state and with an e f f e c t i v e path length deter-mined by ^ . Therefore i t would seem possible to measure I q (using a high scavenger concentration) and I f o r a given s o l u t i o n , k calculate 7"* from the known rate constant (e Q r i + S S") and thereby determine the primary r a d i o l y t i c y i e l d of the hydrated electron. In practice there are two main compli-cations. ( i ) The reference Cerenkov l i g h t I i s a d i f f i c u l t o quantity to measure. A very high scavenger concentration ( ^ 5M) i s needed to reduce the l i f e t i m e of any solvated - 1 1 electron to < 10 sec or to prevent t h e i r formation. At high i o n i c concentrations, both the density and the r e f r a c t i v e index change and thereby a l t e r the number of - 7 1 -Cerenkov photons created as well as the angle at which they are produced. Thus an accurate reference would not be obtained. Experimentally, there was d i f f i c u l t y i n obtaining reproducible r e s u l t s , due perhaps to the neces-s a r i l y highly structured water at these concentrations of solute. ( i i ) The basic equation ignores "presolvation scaveng-ing". The decrease i n Cerenkov reabsorption i s due not only to the rapid reaction of the electrons but also to the f a c t that the electrons may react with certain solutes before becoming hydrated. This presolvation scavenging 2$ i s reported to be an exponential function of scavenger concentration with those solutes capable of t h i s behavior. Data are reported here f o r n i t r a t e (a strong presolvation scavenger), acetone (a weak presolvation scavenger) and hydrogen ion which does not exhibit any presolvation scaven-ging-Accordingly, the r a d i o l y t i c y i e l d G i n the Equation 11-10 has been s p l i t into two parts, G = G q f , where f i s the presolvation scavenging factor and G Q i s the primary r a d i o l y t i c y i e l d i n the absence of any presolvation scaven-ging. Both f and are functions of scavenger concen-t r a t i o n . The e a r l i e r equation i s rewritten as log I M = log I Q M - Go & 10R / ( f T ) (11-16) A plot of log I vs f T f o r a series of solutions should -72-thus be l i n e a r and have a slope equal to Go(£l0R<(/L) and an intercept equal to log ( 1 D ). The numerical value m fo r f was obtained from the data of Wolff et a l . 2 ^ The l i f e t i m e was calculated from the scavenger concentration and the rate constant f o r the hydrated electron at these scavenger concentrations. The plots of the experimental data were a l l reasonably l i n e a r , the best l i n e being obtained by a l e a s t squares a n a l y s i s . Thus, the treatment seems to be j u s t i f i e d . Values f o r Gft and I f o r each set of solutions were o obtained. A more basic treatment of the data was carried out u t i l i z i n g the d i f f e r e n t i a l form of Beer's Law. d i - -oa(x)c(x)dx (11 - 17 ) A CRS equation i s written to include the s p a t i a l dependence of the Cerenkov l i g h t function L(x) by including a production term. d l = L(x)dx - ocI(x)c(x)dx (11-18) I(x) i s the i n t e n s i t y of the Cerenkov l i g h t at any depth x, c(x) i s the concentration of absorbing species at depth x, and <X i s a p r o p o r t i o n a l i t y constant including the natural ex t i n c t i o n c o e f f i c i e n t . The r a d i a t i o n stationary state imposed upon the absorbing species allows the concentration -73-c(x) to be expressed i n terras of the dose f u n c t i o n D(x) c(x) = D ( x ) (10GT/L) (H-19) where D(x) has the a n a l y t i c a l form determined e a r l i e r . Thus C(. = 2 3 . 0 3 £ ^ r / L ( 1 1 - 2 0 ) Equation 1 1 - 1 8 , rewritten as dl/dx +*'D(x)I(x) - Ux) ( 1 1 - 2 1 ) i s immediately recognizable as a l i n e a r f i r s t order d i f -f e r e n t i a l equation with the s o l u t i o n -/^'D(x)dx ; f /<*'D(s)ds I(x) = e J J L(x) e*b dx ( 1 1 - 2 2 ) Equation 1 1 - 2 2 can then be solved f o r the Cerenkov in t e n - • s i t y at x = , the depth at which no more dose i s depos-i t e d . No correction f a c t o r $ f o r the e f f e c t i v e pathlength i s necessary as the functions D(x) and L(x) are used d i -r e c t l y . The a n a l y t i c a l expressions f o r L(x) and D(x) are normalized such that 3 C lt'£ L(x)dx = I Q ( 1 1 - 2 3 ) i f . ? d )(x)dx = R M ( 1 1 - 2 4 ) R„is the maximum r a d i a t i o n f l u x as determined by dosimetry. -74-I i s the reference Cerenkov obtained from the intercept o of the l o g I vs f r p l o t s . In order to solve Equation 11-22 an i n d e f i n i t e i n -t e g r a l J D(s)ds must be evaluated. To do t h i s D(s) i s expanded and then integrated a n a l y t i c a l l y . e" Y = 1-y 2 + y V 2 J - y 6 / 3 ! + ' - y3°/l5T. (11-25) ix - 6 0 . 0 1 with y equal to j 85.5 J J D(s)ds = 2.60 J e " y 2 d y 85.5 (11-26) 0 = 2.60 • 85.5 *• (y-y 3 /3+yV5 .2I - ; (II-27) = xPAN(x) S u f f i c i e n t terms were used i n the expansion to give nearly as good a f i t to the dose-depth curve as d i d the analyt-i c a l expression, the "goodness" of f i t being determined by a non-linear l e a s t squares i t e r a t i v e f i t . The s o l u t i o n to the d i f f e r e n t i a l equation becomes - N « ' / e « ' D(x)dx a d I(x) = e ^ f N'L(x)N "CA'XPAN(X ) dx ° / (11-28) Equation 11-28 i s solved using a computer program (UBC SQUANK) to evaluate the i n t e g r a l s from 0 to a d u t i l i z i n g Simpson's Rule. For a set of f T s or 7-^ ., ranging from .25 X 1 0 " 1 0 to 1.6 X 10" 1 0, the Cerenkov i n t e n s i t y I(x) was calculated f o r G values of 3 . 0 , 3 i 5 , 4 . 0 , 4 . 5 , 5 . 0 . o The calcu l a t e d r e s u l t s along with the experimental points - 7 5 -were p r e s e n t e d a s a f a m i l y o f c u r v e s , and t h e a c t u a l v a l u e f o r G Q was t a k e n t o c o r r e s p o n d t o t h e c a l c u l a t e d c u r v e w h i c h b e s t f i t t e d t h e e x p e r i m e n t a l d a t a . The v a r i a t i o n o f t h e Cerenkov i n t e n s i t y a c r o s s t h e c e l l i n t h e esse where r e a b s o r p t i o n o c c u r s can a l s o be c a l c u l a t e d by t h i s method and i s shown f o r a h y p o t h e t i c a l case i n F i g u r e 18 , -76-- 7 7 -Results: The primary r a d i o l y t i c y i e l d G Q f o r the hydrated electron was calculated from CRS measurements of p e r c h l o r i c acid solutions and of acetone and n i t r a t e solutions under a c i d i c , neutral and basic conditions. In . a l l cases G ' o was obtained from calculations u t i l i z i n g the d i f f e r e n t i a l form of Beer's Law as just described. I was obtained from the intercept of a plot of log I vs. f f ( o r f ) , these p l o t s being reasonably l i n e a r . T y p i c a l data and r e s u l t s are given f o r the case of n i t r a t e ion + 1 M NaOH, Figure 1 9 i s a p l o t of log I vs. f T , and Figure 20 i s the family of curves calculated assuming various G Q values as well as the experimental points. D e t a i l s f o r the p a r t i c u l a r solutions follow, i . P e r c h l o r i c Acid: As the hydrogen ion i s not a presolvation scavenger, the l i f e t i m e T , rather than f T , i s the abscissa on the p l o t s . The l i f e t i m e was determined using the rate con-1 0 2 7 stant of 1.2 X 10 , which i s thought to apply over the range of 0.5 to 5.0 M. There was some deviation from l i n e a r i t y at the extreme points (0.50M and 2.00M). The deviation of the 0.5M point was a t t r i b u t e d to either f a i l -ure of the s h o r t - l i v e d absorber approximation or to a 10 s h i f t i n the rate constant from 1.2 X 10 toward the low concentration value of 2.3 X 10*^. This low concentration Log I v s . f T . p l o t s f o r t h r e e of t h e n i t r a t e s o l u t i o n s w i t h a d d i t i v e s as i n d i c a t e d -80-point would be brought onto the l i n e i f the rate constant were 1.4 X 10"*"^  (actually within Wolff et a l ' s experi-mental e r r o r ) . Figure 21 i s the log I vs. f f p l o t . i i . Acetone; The acetone concentrations ranged from 0.6 to 1.5M. Using the appropriate presolvation scavenging fac t o r s 27 and the high concentration rate constant , these concen-t r a t i o n s correspond to l i f e t i m e s of 1.2 to 2.9 X 10""^sec and f7 values of 0.43 to 1.90 X 10" 1 0sec. The solutions were maintained at various OH- and H + concentrations,and at constant i o n i c strength with NaC10^t The calculated G values are coll e c t e d i n Table VII. The addition of o large amounts of base (IM 0H _) does not increase the G 0 value which has a value of 4.2 f o r a l l the acetone s o l u -tions* The log I vs. f f plots are shown i n Figure 22. i i i . N i t r a t e : The n i t r a t e concentrations ranged from 0.35 to 0.60M. 10 2 7 The high rate constant of 2.0 X 10 and the strong presolvation scavenging c h a r a c t e r i s t i c s necessitated the lower concentration. Ionic strength was not controlled as the addition of IM NaCIO to a neutral s o l u t i o n did 4 not a l t e r the G 0 value. The y i e l d was found to be inde-pendent of OH*" or H + concentration and had an average value of 3.9, as indicated i n Table VII* Typical log I vs. f Y plots are shown i n Figure 19. 2.30-r -81->» •p •H w c <u • P c t—i > o c CD f-t 0) o CD Xi -p « H O to o 2.20* 2.104 2.004 1.90-4 1.8Ct 1.701 1.6CH 1.50t 1.401 5.0 10.0 r U i o 1 1 ) (f=i) F i g u r e 21 Log I v s . L i f e t i m e f o r t h e S e r i e s o f P e r c h l o r i c A c i d S o l u t i o n s . - 8 2 -Table VII I n i t i a l hydrated electron y i e l d s f o r various solutions as determined by CRS measurements Solution G 0 ( e - q ) * G 0 ( e 5 q ) * * Perchloric Acid 4.0 A.3 (612 nm) Nitrate (633 nm) no additives 3.6 4.1 IM NaOH 3.4 3.8 IM NaC104 3.5 4.0 0.1M HC1&. 3.5 3.9 IM Me OH *• 3.7 4.2 Acetone (612 nm) IM NaC104 4.3 4.2. IM NaOH 4.3 4.3 0.1M NaOH + 0.9M NaClO. 4.1 3.9 .01M NaOH + .99M NaClOjL 4.4 4.4 0.1M HCIO^ + .9M NaClO^ 4.0 4.0 3.9 + .4 4.1 ± .2 * Calculated from the integrated form of Beer's Law (Equation II-9). Calculated from the the d i f f e r e n t i a l form of Beer's Law (Equation 11-28). -6*3-fr (xio 1 0) F i g u r e 22 Log I v s . f T P l o t s f o r a S e r i e s o f Acetone S o l u t i o n s -84-Discussion A primary r a d i o l y t i c y i e l d of hydrated electrons was determined f o r a number of solutions of various pH. The y i e l d s were calculated using both the integrated form of Beer's Law (Equation 11-16)^ using the correction f a c t o r ^and the d i f f e r e n t i a l form of the law (Equation 11-28), and are presented i n Table VII. The integrated form i s a reasonable representation of the physical s i t u a t i o n , although the d i f f e r e n t i a l form gives a somewhat more con-s i s t e n t r e s u l t , and i t i s t h i s l a t t e r set of data which i s subsequently discussed. Before the true s i g n i f i c a n c e of the measured y i e l d of 4*1 ± »2 can be assayed a number of points must be considered. How reasonable are the incorporation of presolvation scavenging f a c t o r s and high concentration rate constants, and how s e n s i t i v e are the calculated y i e l d s to such parameters as e , I and R^x? ( i ) Presolvation Scavenging Presolvation scavenging i s a r e l a t i v e l y new concept introduced into r a d i a t i o n chemistry by Hamill to account f o r some anomalous experimental f a c t s . Although the Lea-Gray model does, the o r i g i n a l spur-diffusion model con-t a i n s no provision f o r any chemical action before solvation. The p r i n c i p a l chemically reactive species rather, are H^G+, e~ q, and OH. In contrast Hamill proposed that the primary species formed upon r a d i a t i o n are H 20 + and e" -85-both unsolvated, which may undergo migration, recombina-t i o n or reaction ( i . e . presolvation scavenging) before becoming solvated i n < 10~**sec. The unsolvated, but thermalized, electron, referred to as dry or mobile, may undergo 100 or so c o l l i s i o n s or i n the time necessary fo r s o l v a t i o n. Thus there i s a competition between solva-t i o n and reaction (presolvation scavenging). e" + S > S~ e ~ * eaq (This competition cannot be described by conventional k i n e t i c s , as the time scale of events precludes d i f f u -sion of the species. ) The properties of t h i s precursor, the "dry" electron, were postulated to be s i m i l a r to those of solvated electrons i n alkanes, the main difference being a low r e a c t i v i t y towards H +. The existence of the dry electron as a chemically reactive species was o r i g i n a l l y proposed to explain c e r t a i n anomalies i n the hydrogen y i e l d . G(H2) was observed to decrease i n a manner not explained by competition k i n e t i c s as solute concentration increased and also did not show the expected pH dependence^ Since i t was f i r s t postu-lated presolvation scavenging has received an increasing amount of attention, and i t appears that evidence i s ac-cumulating i n support of the concept. 51 Czapski observed that the r a t i o k 2/k^ (k 2 i s the - 8 6 -rate constant f o r e~ q + S, and k^ i s the rate constant for e~q + H +) varies with scavenger concentration. The v a r i a t i o n can be explained i f there i s a hydrated electron precursor which reacts with most scavengers other than H +- i . e . the data can be r a t i o n a l i z e d n i c e l y by p r e s o l -vation scavenging. An a l t e r n a t i v e explanation, however, i s that the v a r i a t i o n s i n the r a t i o k2/k-^ r e s u l t from time dependent rate constants. Further evidence i n support of Hamill's concept i s 28 presented by V/olff, et a l . , working on the picosecond time-scale of post-radiation e f f e c t s . These workers found that the y i e l d of hydrated electrons i s constant over a hydrogen ion concentration of 0.1 to 5.0M, but that the y i e l d decreases sharply when other electron scavengers are used. These r e s u l t s can be explained by scavenging of dry electrons. The data show that n i t r a t e i s a strong presolvation scavenger while acetone i s a l e s s e f f i c i e n t one. Recent competition s t u d i e s ^ 2 have shown that compounds reduce the y i e l d of hydrated electrons to an extent which does not correlate with t h e i r reaction rates with the hydrated electron. Such r e s u l t s i ndicate that the el e c -trons have the a b i l i t y to react before hydration. Direct observation of the hydrated electron y i e l d also indicates that the decrease in t h i s y i e l d does not correlate with 27 r e a c t i v i t y towards the hydrated electron. -87-53 Other workers have also observed that compounds with low r e a c t i v i t y towards e~ are capable of decreasing aq the y i e l d of solvated electrons. In these CRS experiments i t was observed that, f o r a given calculated l i f e t i m e of the hydrated electron, the measured Cerenkov l i g h t was not the same f o r solutions of the same hydrated electron l i f e t i m e but with a d i f f e r e n t scavenger. See Table VIII. The amount of l i g h t trans-mitted through the solutions was i n the order of H+<. acetone<NO^ . The decrease i n Cerenkov l i g h t transmit-ted agrees with Hunt's measurements of the r e l a t i v e pre-solvation scavenging c a p a b i l i t i e s of these three species. In the treatment of the data presented here, the p r e s o l -vation factors f o r acetone and n i t r a t e obtained by Wolff, 28 et a l . have been used i n the c a l c u l a t i o n of the rad i o -l y t i c y i e l d . Using t h i s f a c t o r the calculated G-values f o r the various scavengers agree reasonably w e l l , as the same i n i t i a l y i e l d i s obtained using as scavengers a non-presolvation scavenger (H +, f = l ) , a weak presolvation scavenger (acetone) and a strong presolvation scavenger ( n i t r a t e i on). A comparison of the plo t s of l o g l vs f f with of log I vs r cannot v e r i f y the correctness of using the presolvation scavenging factor because f and T change with concentration in the same sense. Thus plo t s of l o g l vs.7 are reasonably l i n e a r , but with a slope which d i f f e r s -88-Table VIII Cerenkov i n t e n s i t y i n solutions of s i m i l a r l i f e t i m e of e~ but with solutes of d i f f e r i n g presolvation scavenging a b i l i t y Scavenger Lifetime Cerenkov Intensity (sec) (mV) HT + 0.83 x 10-10 51 NO3 0.83 x H T 1 0 I78* H + 1.67 x l O " 1 ^ 22 Acetone I.67 x 10" x u 33 H + 1.1 x l O - 1 ^ 35: Acetone 1.2 x 10"11? 50.5 NO3 1.25 x 10" 1 0 103* * Corrected f o r wavelength d i f f e r e n c e . -89-markedly from that of the l o g l vs f T p l o t . I f the fac t o r i s not included i n the CRS calculations the G_ (e~ ) value o aq obtained decreases to the neighborhood of 3.4. (See Figure 23 f o r a CRS simulation f o r the n i t r a t e ion i n which no such fa c t o r i s included.) Thus not including a pres o l -vation scavenging fa c t o r i n the c a l c u l a t i o n creates d i s -crepancies between the y i e l d measured with the various scavengers. While the data seems to be adequately handled by the i n c l u s i o n of presolvation scavenging, i t must be men-tioned that there are a l t e r n a t i v e explanations which at leas t p a r t i a l l y explain some of the anomalies mentioned e a r l i e r without invoking a new r a d i o l y t i c species, the 21b dry electron. Schwarz has used time dependent rate constants to p a r t i a l l y explain the lower G (e~ ) observed o aq by Wolff i n acetone solutions. According to him the r e -action of acetone should show a rate decrease as the d i f -f usion gradient i s established. Non-diffusion controlled reactions should also show a time-dependence as a concen-t r a t i o n gradient i s achieved. In t h i s case, a reaction between oppositely charged species (H + and e~ ) should aq show a rate increase with time. In t h i s l a t t e r case, the amount of reaction occurring during the pulse and the dead time of the system would be underestimated, while that i n the acetone case would be overestimated, i . e . , the electron y i e l d s should be more nearly the same. -90-- 9 1 -In a l a t e r paper C z a p s k i ^ has pointed out that Hunt's r e s u l t s could be r a t i o n a l i z e d by postulating the existence of encounter p a i r s . In concentrated i o n i c solutions some solvated electrons w i l l be formed d i r e c t l y as encounter - 1 2 pairs (e~ '--S). I f the l i f e t i m e of the pair i s < 1 0 sec the i n i t i a l electron y i e l d would be underestimated. Between encounter pairs and time dependent rate con-stants the need f o r a scavengable precursor to the hydrated electron i s obviated, although there i s no evidence to r u l e out such a species. Conclusive evidence as regards presolvation scavenging must await further experimental data. Even then the question as to the nature of the precursor may well remain unsettled. As regards the CRS data, the use of the f a c t o r f i s v a l i d a s . i t appears to adequately represent the physical r e a l i t y regardless of whether the decrease i n the i n i t i a l solvated electron y i e l d i s a r e s u l t of presolvation scavenging, encounter p a i r s or time-dependent rate constants. ( i i ) Rate Constants In both treatments of the CRS data, G Q f o r the hy-drated electron was obtained from calculations involving T, the l i f e t i m e of the electron. This l i f e t i m e was c a l -culated from the bimolecular rate constant f o r e + S aq and the concentration of the solute such that T= l / k g ' t ^ Bimolecular rate constants for the hydrated electron have - 9 2 -been measured on the microsecond time scale and are rea-sonably well documented-i-^aRecently these same rate con-stants have been measured on the picosecond time scale 27 i n solutions of concentrated scavengers. The "constants" were observed to vary by as much as a factor of two from those measured on a slower time scale. (See Table IX) The reasons f o r the differences are not e n t i r e l y clear although several explanations have been advanced. An i o n i c strength e f f e c t would seem to explain the d i r e c -t i o n s of the change. Rate of reaction between oppositely charged ions (H + + e ) should decrease with increasing i o n i c strength while that between l i k e charged ions (NO^ + e~ q) should increase. However, there i s no observed change i n k 2 (H + + ®aq) as the j^ H J i s increased from 0.5 to 27 28 5.0M or when NaClO^ i s added.'' This invariance argues against a simple i o n i c strength.effect. Alternate explanations are spur reactions with a 8 -1 f i r s t order rate constant of 4-7 X 10 sec, (these reactions occurring independently) and i o n i c atmosphere e f f e c t s (the atmosphere being not f u l l y relaxed within the reaction time) and, l a s t l y , time dependent rate constants. In these CRS experiments the high concentration rate constants have been used i n the treatment of the data. The experiments were performed i n the scavenger concen-t r a t i o n range covered by these rate constants and the Solute ' k ( d i l u t e ) -1 i (M sec" 1) G°(e- d) k (concentrated) (M^sec" 1) G°<eaVd q Concentration range (M) H+ NOj 2.3 X l O l O 3 i . i x i o l o a n b 2.2 7.1 10 c 1.2 X 10 2.0 X 1 0 l o b 4.3 3 . 9 0.5 - 5.0 0.1 - 0.75 Acetone 7.6 X 10 y 5.6 X 10 9 a 5.2 7.1 9.5 X 1 0 9 b 4.2 0.2 - 1.75M Table IX I n i t i a l hydrated electron y i e l d s , a s calculated using r a t e c o n s t a t s f o r ' & d i l u t e and concentrated scavengers ^ i a. Reference 10a b. Reference 27 c. Reference 26 d. This work calculated using the in d i c a t e d r a t e constant and the d i f f e r e n t i a l form of Beer's Law. - 9 4 -time domain being probed was the 10~*^ to 1 0 " ^ second range. Consequently these rate constants seem to be the appropriate ones to use. Further j u s t i f i c a t i o n came i n the plot of l o g l vs. 1/ |H+J where the 0.5M point which f e l l o f f the l i n e could be brought back on by a s l i g h t s h i f t of the high concentration rate constant toward the d i l u t e solution value (1.2 to 1.4 X 10*^). Using these rate constants the G° values ar r i v e d at from the various solutions a l l agree reasonably w e l l . (See Table VII). However,as can be seen i n Table IX, i f the d i l u t e solutions k2*s are applied large discrepancies develop between the various solutions.. An i n i t i a l electron y i e l d o f ~ 7 seems completely out of l i n e with the M sec y i e l d s as well as the various i n i t i a l y i e l d s predicted by the current theories. Use of the high concentration rate constants would therefore seem well j u s t i f i e d . ( i i i ) Further Considerations In the determination of the r a d i o l y t i c y i e l d using the d i f f e r e n t i a l form of Beer's Law (Equation I I - 9 ) , an e x p l i c i t value of l o i s necessary. Because of the large experimental uncertainty i n the d i r e c t measurement of I 0 , the value used i n these calculations was obtained from the intercepts of the various l o g l vs. f T p l o t s . In t h i s manner much of the uncertainty was averaged out as each point was the average of a minimum of f i v e meas-urements, and each l i n e was determined by a minimum of -95-four points. The calculated Cerenkov i n t e n s i t y f o r a given G Q does not correlate at a l l with the experi-mental points i f I i s either greater or l e s s than 5% of the intercept value. The l a s t factor to be discussed, which enters the c a l c u l a t i o n of G r t(e~ ), i s the extinction c o e f f i c i e n t o aq ' for the hydrated electron at the wavelength of the experi-ments. Two considerations are of importance, the wave-length of the l i g h t and the actual value of e at that wavelength. The interference f i l t e r as operated had a band-pass which i s bell-shaped with a width-at-half-height of 15nm. The s i t u a t i o n i s complicated by the f a c t that as the wavelength decreases across t h i s bandwidth, the i n t e n s i t y of Cerenkov l i g h t increases, while the extinc-t i o n c o e f f i c i e n t of the hydrated electron decreases. This means that the e f f e c t i v e £ should be s l i g h t l y l e s s than that corresponding to the center of the band-pass. (Fortunately the photodiode has a r e l a t i v e l y constant s e n s i t i v i t y over the range i n question.) Calculations indicate (effective^ i s < 2% l e s s than the nominal ex-t i n c t i o n c o e f f i c i e n t . The actual values of the extinc t i o n c o e f f i c i e n t s used were 1.33 X l c A M-icm" 1 (612 nm) and 1.44 X 10^ M-1cm~1 (633 nm), and are those reported by Fielden and Hart*** These extinct i o n c o e f f i c i e n t s d i f f e r from those derived 55 from Keene's measurements of G£ . Using a ^u-sec G value of 2.65, e x t i n c t i o n c o e f f i c i e n t s of 1.19 X lO4* and - 9 6 -1.32 X 10/+M'1cm*"1, at 612 and 633 nm resp e c t i v e l y , are obtained from Keene's data. The v a r i a t i o n i s e s s e n t i a l l y l i n e a r and, thus, a 7% increase in e would r e s u l t i n a 7% decrease i n G Q. This l i n e a r dependency i s also what would be predicted from the integrated form of Beer's Law (Equation I I - 9 ). The ef f e c t of using Keene's extinc-t i o n c o e f f i c i e n t s instead of Fielden and Hart's would thus be to increase a G Q of 4.0 to 4«4» It i s f e l t that Fielden and Hart's values are the better ones to use because t h e i r reported absorption spec-trum has the same r a t i o of £ (Xmax)/6 {57# nm) as a number of other workers, while Keene's has a s l i g h t l y lower r a t i o . Also using these values allows a d i r e c t comparison with other recent measurements. The effect of added solutes i n the 1-2M range on the absorption spectrum of the hydrated electron i s thought to be n e g l i g i b l e . High concentrations (15M) of added 57 solutes w i l l s h i f t the spectrum to lower wavelengths, but added solutes i n the 1-2M range do not seem to make 26,36 a measurable change i n the pos i t i o n of the band maximum. The extinction c o e f f i c i e n t i s also taken to be unchanged. -97-Within the l i m i t a t i o n s of the various fac t o r s d i s -cussed previously, the i n i t i a l r a d i o l y t i c y i e l d of hydrated electrons for the various solutions of scavengers were calculated and are tabulated i n Table VII. For a given scavenger, the agreement between the G Q calculated for the various scavenger solutions i s quite good, with the standard deviations being much smaller than the error r e s u l t i n g from the various assumptions (estimated to be t 10%). It i s d i f f i c u l t to determine whether the d i f f e r -ence between the n i t r a t e and acetone solutions i s t r u l y s i g n i f i c a n t . I t i s possible that the true acetone concen-t r a t i o n was l e s s than the nominal one because of v o l a t i l i t y or some type of condensation reaction (possibly base-catalyzed). I f so, the calculated G 0 would be greater than the actual one. This i s thought to be more l i k e l y than the r a d i o l y t i c y i e l d being dependent upon the actual scavenger. An o v e r a l l r a d i o l y t i c y i e l d f o r the hydrated electron of 4«1 t .4 was calculated by averaging the y i e l d s . The error represents the cumulative e f f e c t s of the various f a c t o r s involved i n the determination, rather than a stan-dard deviation. A G Q value of 4.1 i s somewhat higher than the 3.2 +0.8 value measured o r i g i n a l l y by CRS under a c i d i c con-41 d i t i o n s . The main difference appears to r e s u l t from the lower dose rate measured i n t h i s work. An i n i t i a l y i e l d of about 4 agrees very well with the y i e l d s measured at -98-30-psec (4.0 i .1) and at 200 psec (4.1 ± 0.1)? 9 However, a l l of these values are at variance with the pre d i c t i o n of spur-diffusion theory (G0=4.7#)and other i n d i r e c t meas-urements which involve either extrapolation of scavenging e g studies, (G Q - 5.0), or extending the l i f e t i m e of the hydrated electron by either adding solute to preclude spur reactions which remove the electron (G * 5.05) 3^ or o lowering the temperature (G Q = 5.0).^ (See Table IV,) O r i g i n a l l y i t was thought that the experimental d i s -crepancies, i n p a r t i c u l a r between Hunt's and Buxton's work,might be a r e s u l t of differences i n experimental conditions i n p a r t i c u l a r of the pH of the solutions. The picosecond work was done under very a c i d i c conditions while the i n d i r e c t measurements were performed under very basic conditions. O r i g i n a l l y there was some question as to whether a d d i t i o n a l hydrated electrons might be formed under basic conditions, however recent work^ 2 at 200 psec i n 1M NaOH solutions agree with the e a r l i e r picosecond work. These CRS measurements confirm the pH independence of the i n i t i a l y i e l d . 1M NaOH added to both the n i t r a t e and the acetone solutions d i d not a f f e c t the G 0 measured. Furthermore these values agree with the G Q measured using perchloric acid as the scavenger. It becomes very apparent that the spur-diffusion model must be alte r e d somewhat i n order to accommodate the more recent experimental data. Recent calculations by supporters'' 9 of the model demonstrate that the hydration -99-time of the hydrated electron i s important on the time scale of the picosecond experiments. I f the rate of hy-dration i s 4 X 10 * * s e c - ^ (hydration occurs i n 2-4 p s ) , ^ a certain portion of the electrons w i l l react before hy-dration, and an i n i t i a l y i e l d of hydrated electrons of 4.0 becomes most reasonable. I f hydration takes longer than a few picoseconds, a y i e l d of lower than 4.0 would be observed i n the picosecond time scale. Conversely, i f the hydration period were less than about 2 picoseconds, the i n i t i a l y i e l d would be greater than 4*0' Such an approach seems to incorporate Haroill's model into the spur-diffusion model, and would seem to go a long way toward r a t i o n a l i z i n g t h e model and various sets of experi-mental data. The hydration time would not a f f e c t nanosecond measurements, which would reasonably be extrapolated back to a hydrated electron y i e l d of about 5. Recently, however, a hydration time of 0.2 ps has been determined by extrapolation by the same workers who report an i n i t i a l y i e l d of hydrated electrons of 4»0« These two pieces of data are mutually exclusive by Schuler's c a l c u l a t i o n s . The s i t u a t i o n at the moment thus seems to be i n a state of f l u x . The most recent experimental measurements of the i n i t i a l hydrated electron y i e l d at 20 psec, 200psec and by CRS disagree with the predictions of the spur-d i f f u s i o n model. No simple modification of the theory adequately represents the data, and i t may be necessary to change one or more parameters. It does appear that -100-Hamill's proposal that the "dry" electron i s capable of reaction i s v a l i d and must be incorporated into the spur-d i f f u s i o n model. More experimental data must be obtained, and the theory reworked allowing f o r time-dependent rate constants, hydration time, and r e a c t i v i t y of the dry electron, among others. Cerenkov Reabsorption Spectroscopy i s probably not the best approach to the problem of i n i t i a l y i e l d s , a l -though the r e s u l t s agree with other measurements on the same time scale. Certain modification to the pulse r a d i o l y s i s system would improve the measurements however. A measurement of the dose delivered by each pulse would be desirable, however t h i s would not correct f o r the con-siderable v a r i a t i o n s i n pulse shape that were apparent from the Faraday cup measurements. A more precise wave-length s e l e c t i o n would also be an improvement, e s p e c i a l l y i f the measurements could be made more clos e l y to X M A X f o r the hydrated electron. In t h i s manner small v a r i a t i o n s i n e. would be minimized; i n p a r t i c u l a r v a r i a t i o n s which might be caused by s l i g h t s h i f t s i n the spectrum. The l i m i t a t i o n to t h i s would be the s e n s i t i v i t y of the photo-diode as the number of Cerenkov photons decreases with increasing wavelength. In p r i n c i p l e the treatment of data could be s i m p l i f i e d by using an empirical value f o r ( £ 10R *7L ) obtained by measurement of I and I f o r a species whose G-value on the picosecond time scale i s -3 0 1 -known and whose e x t i n c t i o n c o e f f i c i e n t i s f i r m l y e s t a b -l i s h e d . No such s p e c i e s e x i s t s , but perhaps the hydrated e l e c t r o n may soon become u s e f u l i n t h i s r e g a r d . -102-Chapter III On the C h i r a l i t y of Solvated Electrons A. General Introduction The second part of t h i s thesis concerns an attempt to detect c h i r a l i t y or "handedness" on the part of the solvated electron through i t s i n t e r a c t i o n with c h i r a l molecules and polarized l i g h t . The question of the o r i g i n of the o p t i c a l a c t i v i t y found i n nature has intrigued s c i e n t i s t s for many years. Two schools of thought have developed with respect to the problem with one advocating pure chance and the other a s p e c i f i c dissymmetric influence as the cause of the ac-62 t i v i t y . The pure chance pathway might involve spontan-eous r e s o l u t i o n i n the p r e b i o t i c stage or natural s e l e c t i o n during the b i o t i c stage. Various dissymmetric influences which have been invoked to explain the a c t i v i t y include ( i ) i n t e r a c t i o n on asymmetric c r y s t a l l i n e mineral 63 surfaces, ( i i ) energy differences between enantiomorphic 64 structures which might r e s u l t i n d i f f e r i n g s o l u b i l i t i e s , ( i i i ) magnetic f i e l d s as of the earth, (iv) c i r c u l a r l y polarized l i g h t and (v) asymmetry of elementary p a r t i c l e s , or some combination of these e f f e c t s . As early as 1894 Van'tHoff 0 5 suggested that asymmetric products might be formed from reactions u t i l i z i n g c i r c u l a r l y polarized l i g h t . Not too long a f t e r t h i s proposal o p t i c a l rotations were reported as r e s u l t i n g from an asymmetric -103-/3 ^0 decomposition of CH^-CH-C-OCaHs. A s y m m e t r i c s y n t h e s e s were reported by other workers§7 Quite recently the syn-t h e s i s of nonracemic helicenes using c i r c u l a r l y p olarized l i g h t has been r e p o r t e d ^ Equal and opposite rotations were obtained with the use of r i g h t and l e f t c i r c u l a r l y polarized l i g h t . These experiments confirm that c i r c u l a r l y p olarized l i g h t can induce o p t i c a l a c t i v i t y , however the source i n nature of the r e q u i s i t e l i g h t i s unclear. 69 The suggestion by Lee and Yang that the p r i n c i p l e of p a r i t y could be v i o l a t e d i n weak interactions increased i n t e r e s t i n the dissymmetric influence school of thought. Simply, the p r i n c i p l e of p a r i t y states that the laws of nature are invariant under space r e f l e c t i o n . Beta decay (a neutron decaying into a proton and an electron) (Equa-t i o n I I I - l ) i s an example of a weak i n t e r a c t i o n , the forces -12 involved being about 10 times as strong as a nuclear p-p i n t e r a c t i o n . n > p + + e ~ + 73 ( I I I - l ) Wu, et a l . 7 ^ confirmed t h i s hypothesis with the observation of an asymmetric angular d i s t r i b u t i o n of electrons r e s u l t -ing from the decay of aligned^°Co n u c l e i (Equation III-2) 6 0 C o » 6°Ni + e" + U ( I H - 2 ) 71 The electrons were found to be left-handed. Lee and Yang explained the non-parity of /S-decay by postulating that neutrinos (and anti-neutrinos) are allowed only one spin -104-72 state. Further work on /3 -decay resulted i n the discovery that the Bremsstrahlung produced by the p p a r t i c l e f o l -lowing strontium decay was c i r c u l a r l y polarized. (Equation I I I - 3 ) 90 90 - ^ c i r c u l a r l y polarized Sr > Y + / 3 Bremsstrahlung ( I H - 3 ) 38 39 polarized Any 7-rays following the /3-decay are also expected to be c i r c u l a r l y polarized to some extent as the nucleus 73 i s l e f t p a r t i a l l y polarized a f t e r the decay. The existence of asymmetry on the elementary p a r t i c l e l e v e l posed the question of whether t h i s asymmetry could be transferred to the molecular l e v e l . Various pathways are possible and are discussed by U l b r i c h t ^ 7 t ) ' ^ P e r h a p s the most obvious route i s that through CP ( c i r c u l a r l y polarized) l i g h t . polarized 7-rays ^ c.p. l i g h t ^ o p t i c a l l y active molecul Other induction mechanisms might involve magnetic e f f e c t s or entropy exchange. Whether secondary electrons produced by the moderation of 7 -rays are polarized i s not known, however, i f so, they might provide another pathway fo r asymmetric induction. Working along these l i n e s experiments have been devised which seek to e s t a b l i s h r a d i a t i o n induced c h i r a l i t y on the molecular l e v e l . U l b r i c h t and Vester 7** synthesized under the influence of r a d i a t i o n a number of compounds containing asymmetric centers and concluded that i f any induced o p t i c a l -105-75 a c t i v i t y existed i t was less than 0.02%. Garay looked f o r p r e f e r e n t i a l destruction of enantiomorphs incubated with radioactive strontium which produces polarized y -rays. He expected that i f an electron i n an o p t i c a l l y active molecule moves i n a h e l i c a l path, i t might i n t e r a c t d i f -f e r e n t l y with either polarized Bremsstrahlung or possibly polarized secondary electrons. Results from these decom-po s i t i o n experiments i n which the Ddsomer decomposed s l i g h t l y more over 18 months were not unambiguous and other s i m i l a r experiments have produced no conclusive 76 r e s u l t s . More recently, along s i m i l a r l i n e s , Garay has measured the a n n i h i l a t i o n of polarized positrons i n D and L amino acids and found that the t r i p l e t states of the positron i s favored i n the D-isomers which i s i n t e r -preted as an i n d i c a t i o n that decay might have been a f a c t o r i n the creation of asymmetry on Earth. 77 E a r l i e r work i n t h i s laboratory sought to use the solvated electrons produced from polarized 7 -rays as the induction agent f o r o p t i c a l a c t i v i t y on the molecular 6 0 l e v e l . Cobalt-60 decays to a stable Ni by emitting two T-rays (1 .33 and 1.17 MeV). Because the decay i s an example of a weak i n t e r a c t i o n these 7-rays are at 71 78 l e a s t p a r t i a l l y c i r c u l a r l y polarized. ' The 7-rays lose energy by the Compton e f f e c t i n which a r e c o i l electron and a scattered photon are ejected. When the r e c o i l electrons are thermalized, solvated electrons are - 1 0 6 -produced. It was postulated'' that the h e l i c i t y of the o r i g i n a l 7-rays was c a r r i e d through to the solvated electron, producing c h i r a l solvated electrons. I t was further pro-posed that electrons of d i f f e r e n t handedness would react d i f f e r e n t l y with each member of a pair of enantiomorphs. A difference i n rate was deemed most probable. The exper-iments consisted of i r r a d i a t i n g d l mixtures i n a Gammacell and then looking f o r p r e f e r e n t i a l destruction of one of the isomers. The desired e f f e c t would be confirmed by the existence of c i r c u l a r dichroism of the i r r a d i a t e d mixture. No p o s i t i v e r e s u l t s were obtained. The experiments could have f a i l e d f o r one or more of several reasons. 1 . The asymmetry of the 7-photon was not transferred to the Compton electron and subsequently to the solvated electron. 2. Solvated electrons of opposite handedness do not react p r e f e r e n t i a l l y with one of a p a i r of enantiomorphs. 3. Spin f l i p p i n g (III - 4 ) i s too rapid to allow e^ q ( 1 ) *=± e j q (r) <m-4> any detection of d i f f e r e n t i a l r a t e s . 4. The basic experiment and means of detection were simply not s e n s i t i v e enough. This present work attacks the same basic problem, but i s designed to eliminate number 1 and to minimize -107-number 3 as reasons for f a i l u r e . Rather than use the Gammacell which requires asymmetry to be transferred a number of times, the solvated electrons were produced from the electron beam from the Febetron. Unless there i s some asymmetry of the so l u t i o n , equal concentrations of r i g h t -and left-handed electrons should be formed. The problem was then approached i n two ways: 1. The k i n e t i c i n t e r a c t i o n of the solvated electrons with o p t i c a l isomers was examined i n an attempt to detect the d i f f e r e n t rate constants assumed i n the 7 - c e l l experi-ments. 2. The i n t e r a c t i o n of these electrons with polarized l i g h t was examined f o r differences i n ext i n c t i o n c o e f f i c i e n t s f o r the r i g h t and l e f t handed solvated electrons. General Experimental Approach The two approaches w i l l be treated separately, although the experimental set-up was s i m i l a r i n both cases. The t o t a l concentration of the hydrated electrons was monitored spectrophotometrically by observation of the absorption of l i g h t from a He/Ne la s e r directed through the s o l u t i o n . At the wavelength of the^laser (632.8 nm) the hydrated electron has an intense absorption band ( e = 1.44X10^M~*cm~^ * 4 ) . The method i s generally termed k i n e t i c laser spectrophoto-metry. The general a p p l i c a t i o n of t h i s method to l i q u i d solutions using the Febetron as the electron source has been developed p r e v i o u s l y ^ , 7 ^ T h e r e are two major compli--108-cations which a r i s e from the use of the Febetron. The dose deposited by the Febetron i s not homogeneous, r e s u l t i n g in a non-uniform concentration of hydrated electrons across the c e l l ; but t h i s was of no consequence because the k i n e t i c s f o r t h i s work were pseudo-first order. As the Febetron can develop a hydrated electron concentration -6 -5 of ~ 10 - 10 M, scavenger concentrations should be -3 ~ 10 M. As a further consequence of the inhomogeneity, a small cross-section of the solut i o n i s monitored to avoid large concentration gradients. This i s generally accomplished through the use of a small pinhole r e s t r i c t i n g 2 the l i g h t to a few tenths of a mm. A phenomenon termed "delayed absorbance" 7 9'^compli-cates the early nanoseconds a f t e r the electron pulse. An apparent increase i n the absorbance occurs. The cause of t h i s phenomenon i s s t i l l unclear, although i t i s most pronounced in regions of high deposited dose and i n solutions with a high hydroxide ion concentration. More w i l l be said about the p r a c t i c a l problems involved i n circumventing the e f f e c t l a t e r . A t h i r d l i m i t a t i o n to k i n e t i c studies using the Febetron 79 81 (but not a problem i n t h i s work) i s a "shock wave" * which passes through the c e l l d i s t o r t i n g the s i g n a l a f t e r several hundred nanoseconds. -109-B. The Reaction of Solvated Electrons with C h i r a l Molecules 1. Introduction The basic assumption i n t h i s approach i s that the solvated electron of a p a r t i c u l a r handedness w i l l react d i f f e r e n t l y with two enantiomorphs. It i s assumed that equal concentrations of r i g h t - and left-handed solvated electrons w i l l be developed by the electron beam. The reaction scheme then can be written as e - s o l (1) + d -±L-> ( i n - 5 ' ; ) e s o i ( r ) + ( i e so x (1) +-1 ( I I I - 6 ) ( H I - 7 ) e" . (r) + 1 — k4~> s o l ( I I I - 8 ) where d and 1 are a pair of o p t i c a l isomers and e~ (1 ) s o l and e ., (r) are solvated electrons of opposite handedness, s o l None of the rate constants are necessarily equal. In the pulse r a d i o l y s i s of one of the o p t i c a l isomers the reaction scheme w i l l be r e s t r i c t e d to either I I I - 5 and I I I - 6 or I I I - 7 and III-& . The decrease i n hydrated electron concentration i s represented as -d fe~P ) + e" (r)"J = ^ f d l f e " (1 )J + k 2 [dj [e"(r)] ( H I - 9 ) dt I f [d3-X> [e~J (pseudo f i r s t order condition) then z d j V l l _ + ^ l = ^ ( e - d ) ] + k 2 l £ e-(r)] (111-10) -110-and e (1+r) = e (1 )exp( -k» t)+ e ( r ) e x p ( - k X t ) O 1 O r <L ( H I - 11) where t = time e- Q(l ) = i n i t i a l cone, of left-handed electrons e 0 ( r ) = i n i t i a l cone, of right-handed electrons e (r) = e (1) = \ e (111-12 ) where e Q i s the measured i n i t i a l solvated electron con-centration. Thus, r - k ' t [e"(l+r)] = e Q / 2 je " 1 (111-13 ) + e and the concentration terms can be replaced by the absorb-ance (the measured quantity). I f k-^  = kg the equation reduces to a simple pseudo f i r s t order k i n e t i c scheme. I f / k 2 a p l o t of logA (or InA) vs. time should not be l i n e a r , but should have a changing slope. I f k^ = 2k 2 , such a plot shows a change i n slope at approximately the f i r s t experimental h a l f - l i f e . In choosing the best system to optimize |k -k \ there 1 ^ are several considerations. ( i ) A spin f l i p p i n g reaction ( I I I - k ) should be i n -cluded i n the reaction scheme. EPR measurements'1"0 of the linewidth of the hydrated electron indicate that the electron has a slow relax a t i o n time of ~10"^se.c. It i s necessary that the electron react before i t s p i n - f l i p s i n the experiments. Therefore, i t s h a l f -e" ( l ) e (r) ( I I I - % ) - I l l -l i f e should be l e s s than about 20 nsec. ( i i ) The solvated electron should not react at every c o l l i s i o n i n order to maximize z^k. A rate constant can be considered as being made up of two parts, an a c t i v a t i o n energy term and a preexponential f a c t o r (Equation H l - 1 4 ) . k - A e Xp(-E a/RT) ( I I I - H ) The a c t i v a t i o n energy f o r most hydrated electron reactions i s about 3-4 kcal/mole - the energy necessary f o r d i f f u s i o n . The differences i n the various rate constants thus r e s u l t s from the preexponential f a c t o r . According to the k i n e t i c theory of gases t h i s factor represents the number of c o l -l i s i o n s m u l t i p l i e d by a s t e r i c factor which allows f o r preferred c o l l i s i o n a l configuration. The scavenger should be a compound towards which the electron i s not overly r e a c t i v e , i . e . one with a small preexponential term, so that the solvated electron may undergo ^ 100 c o l l i s i o n s before reaction. The i d e a l second order rate constant 8 - 1 _ i 7 i s ^ 1 0 M sec , a f a c t o r of 1 0 l e s s than a d i f f u s i o n controlled rate constant. A scavenger concentration of ~.3M thus gives an electron h a l f - l i f e of ^ 20 nsec and allows the r e q u i s i t e number of c o l l i s i o n s within the time period. ( i i i ) The o p t i c a l l y a c t i v e center should be as close to the reaction s i t e as possible so as to maximize the asymmetry which the solvated electron experiences. In summary the i d e a l s i t u a t i o n should be a concen--112-t r a t e d s o l u t i o n o f a r e l a t i v e l y u n r e a c t i v e compound whose o p t i c a l l y a c t i v e c e n t e r i s a l s o the r e a c t i o n s i t e . In p r a c t i c e compromises must be made as a r e s u l t o f a v a i l a -b i l i t y , p u r i t y , s o l u b i l i t y , e t c . - 1 1 3 -2. Experimental  General The basic techniques f o r k i n e t i c pulse r a d i o l y s i s studies of l i q u i d s using the Febetron as the electron 79 source has been developed e a r l i e r . B r i e f l y , the electron beam passes through a O.OOl" s t a i n l e s s s t e e l electron window into a c e l l containing the l i q u i d . The hydrated electron (or any other appropriate species) concentration i s monitored with a He/Ne la s e r (632.8 nm) directed through a narrowly defined cross section of the solution perpen-d i c u l a r to the electron beam. (It i s desirable to r e s t r i c t the area of the solution because the dose i s deposited non-uniformly.) The absorbance of the l i g h t i s propor-t i o n a l to the concentration of the absorbing species as given by Beer's Law A = -log 1/IQ = €bC where A = absorbance I = l i g h t transmitted I = reference l i g h t (no absorption) € - decadic molar extinc t i o n c o e f f i c i e n t b - pathlength (cm) C = cone, of absorbing species (molar) The pathlength i s determined by the width of the electron beam entering the sample. - I H -Th e small cross section of solution i s obtained with a "pinhole" r e s t r i c t i n g the laser beam as i t exi t s from the c e l l . D i f f r a c t i o n rings were removed by an adjustable i r i s . Lead removed unwanted x-rays. A lens focused the laser beam onto the detector, a photomultiplier. An i n t e r -ference l i n e f i l t e r f o r 632.8 nm (Baird Atomic Inc. Inter-ference type B II) placed d i r e c t l y i n front of the photo-m u l t i p l i e r removed stray l i g h t . The s i g n a l from the photo-m u l t i p l i e r was transferred to an oscilloscope where the trace was photographed on 10,000 ASA Polaroid f i l m (type 410). Cables were 50.A- impedance and doubly shielded. Undesir-able r . f . noise was minimized by an aluminum cap over the c e l l and aluminum wrapping of the Febetron. The basic set-up i s shown i n Figure 2/^  P a r t i c u l a r s C e l l : Two c e l l s were used f o r t h i s work. The f i r s t c e l l has been described elsewhere? 9 A second c e l l (Figure 2.5) was designed and used f o r the majority of these experi-ments. The c e l l was constructed from s t a i n l e s s s t e e l with a c y l i n d r i c a l sample compartment (13 mm width, 30 mm length). The c e l l was emptied and f i l l e d through ports brazed to the c e l l and machined to f i t the glass flow system. The c e l l contained a 0.001 s t a i n l e s s s t e e l e-lectron window which was f i t t e d with a s e r i e s of s l i t covers to define the beam width (and thus pathlength). B A G PSB • N L l _ I E x p e r i m e n t a l S e t - u p f o r 0- O s c i l l o s c o p e B- A m p l i f i e r -L- He/Ne l a s e r C- R a d i o l y s i s c e l l Febe- F e b e t r o n 1- I r i s t o remove d i f f r a c t i o n G- l e a d s h i e l d i n g S- S h u t t e r T r i g - T r i g g e r f o r F e b e t r o n F i g u r e 24 the K i n e t i c L a s e r S p e c t r o p h o t o m e t r y E x p e r i m e n t s PSB- Power s u p p l y f o r a m p l i f i e r N- N e u t r a l d e n s i t y f i l t e r s D- F o c u s i n g l e n s F- I n t e r f e r e n c e f i l t e r (632.8) r i n g s PM- P h o t o m u l t i p l i e r PSP- Power S u p p l y f o r Pin T- 50 t e r m i n a t o r A -500 MHz f i l t e r -116-Figure 25: Kinetic Laser Spectrophotometry C e l l a. Entrance and exit ports. b. Electron window - variable s i z e s . c. C e l l compartment. d. Windows f o r analyzing l i g h t - covered by r e s t r i c t i n g covers as i n e. -117-S i l i c a windows were at either end of the c e l l to allow the analyzing l i g h t to be transmitted. The c e l l was af-fix e d to a flange set 5 cm back from the electron tube. An aluminum cover enclosed the c e l l and contained a move-able pinhole ( 0 . 0 1 3 5 " diameter) which r e s t r i c t e d the analyzing l i g h t emerging from the c e l l . This c e l l design minimized "delayed absorbance" by having a larger pinhole and by being further removed from the electron source. The electron beam was delivered by the Febetron which was equipped with a model 5 5 1 0 electron tube. (A 5 5 1 5 tube was used f o r the CRS measurements.) The 5 5 1 0 tube has a s l i g h t l y d i f f e r e n t dose depth p r o f i l e than the 5 5 1 5 tube (Figure 3 ) . The energy i s les s concentrated on axis, r e s u l t i n g i n a lower dose rat e . The change was made i n an e f f o r t to minimize "delayed absorbance." The analyzing l i g h t was supplied by a He/Ne la s e r . Two models were used: a Spectra Physics model 1 2 0 having a power r a t i n g of 5mV and a beam diameter of 0 . 6 5 m m ( l / e th width) and a Spectra Physics model 1 5 0 having a power r a t i n g of 3mV and a s l i g h t l y wider beam diameter. The model 1 2 0 i s plane polarized while the model 1 5 0 i s not. No difference i n the decay k i n e t i c s as monitored by eithe r l a s e r s was detected. -118-E l e c t r i c a l Detection: The l i g h t transmitted through the solution was meas-ured with a photomultiplier RCA 1P28 consisting of a photo-cathode (spectral response of S5), a nine-stage l i n e a r dynode chain and a 50/1 anode load. The photomultiplier was encased i n lead except f o r a narrow entrance port f o r the l i g h t . Normal operating voltages were 550 to 700 V supplied by a Fluke model 412 B power supply. Neu-t r a l density f i l t e r s ( s i l v e r e d mirrors from O r i e l Optics) were used to adjust the i n t e n s i t y of the signal and thus the amount of current drawn from the photomultiplier (1 milliamp). Under these conditions the photomultiplier was l i n e a r and had a r i s e time of < 2 nsec. On occasion the RCA 1P28 photomultiplier was replaced by a Hammamatsu (HTV) R213 tube having an S20 spectral response. The s i g n a l from the photomultiplier was monitored with an osc i l l o s c o p e . Two models were used, a Hewlett-Packard model 183 and a Tektronix model 454. In general the HP 183 was used as i t has a somewhat f a s t e r r i s e time than the Tektronix (1.2 nsec vs. 2.4 nsec) and i t s ver-t i c a l a m p l i f i e r i s equipped with a 50fl impedance. Flow System: An a l l glass flow system was used to f i l l the c e l l and to replenish the solu t i o n . The solution was degassed by bubbling with helium gas. (Degassing was not c r i t i c a l because of the f a s t decay times involved i n these experi--119-ments. ) The f l o w system was kept under a s m a l l p o s i t i v e p r e s s u r e o f t h e gas and t h e s o l u t i o n i n t h e c e l l c o u l d be changed by m e r e l y pumping f r e s h s o l u t i o n t h r o u g h . Care was t a k e n t h a t no a i r b u b b l e s were l e f t i n t h e c e l l . The s o l u t i o n was g e n e r a l l y changed a f t e r e v e r y p u l s e . -120-Materials: Solvents: Laboratory d i s t i l l e d water was r e d i s t i l l e d from a c i d i f i e d potassium dichromate. The n-amyl alcohol (Fisher C e r t i f i e d ) was refluxed with, and then f r a c t i o n a l l y d i s t i l l e d from, 2, 4 - d i n i t r o -phenylhydrazine and s u l f u r i c acid under a nitrogen atmos-phere. The middle f r a c t i o n (b.p. 133.5 - 134.0° C) (Handbook of Chemistry + Physics, 13#°C) was c o l l e c t e d and stored under nitrogen over molecular sieves (1/16", type 5A). The prim.act. amyl alcohol (D-2-methyl-l-butanol; K + K) was f r a c t i o n a l l y d i s t i l l e d from a c i d i f i e d 2,4--dinitrophenylhydrazine. The f r a c t i o n b o i l i n g at 125 -126°C was collected and stored under nitrogen. (Handbook of Chemistry + Physics, b.p. = 12&°C) The o p t i c a l r o t a t i o n of the d i s t i l l e d material as measured with a Perkin Elmer 141 Polarimeter at 25°C and i n 0.10 dm c e l l was -O.46O0 as compared with -0.455 for the u n d i s t i l l e d . Ethyl alcohol ( 1 0 0 % and 95%) was used without further p u r i f i c a t i o n . None of the solvents used exhibited s i g n i f i c a n t re-a c t i o n with solvated electrons during the f i r s t 75-100 nsec a f t e r being subjected to an electron pulse, i n d i c a t i n g that t h e i r purity was adequate f o r the proposed experi-ments. - 1 2 1 -Solutes: The solutes were obtained from the sources indicated below i n the best grade a v a i l a b l e , and were used without further p u r i f i c a t i o n . (l ) - 3-chlorobutyric a c i d - Norse Chemical Laboratories (dl ) - 3 - ° chlorobutyric a c i d — Norse Chemical Laboratories 1-mandelic acid - A l d r i c h Chemical Co. dl-mandelic acid - A l d r i c h Chemical Co. 1-glutamic a c i d , disodium s a l t - A l d r i c h Chemical Co. 1-camphor - Fisher d-camphor - Fisher -122-Several modifications were made to the detection system.in order to improve the signal/noise i n order to observe smaller absorbances. These changes arose out of a need to minimize "delayed absorbance", which i s more predominent i n areas of high dose. If the absorbance was monitored i n a low dose region (toward back of the c e l l ^ 1 mm from front face), delayed absorbance was l e s s of a problem. To measure the smaller absorbances i t was necessary to increase the gain of the detection system. This was accomplished by using a pulse a m p l i f i e r . As 79 laser r i p p l e was also amplified, a tuned c i r c u i t was added to eliminate t h i s undesirable noise. A shutter mechanism was included i n the o p t i c a l t r a i n to r e s t r i c t i n time the l i g h t s t r i k i n g the photomultiplier and thus allow larger anode currents to be drawn without photo-m u l t i p l i e r fatigue. The tuned c i r c u i t * ( F i g u r e 26) consisted of a variable capacitor adjusted so as to "tune out" the l a s e r r i p p l e of about $00 MHZ. The c i r c u i t was tuned v i s u a l l y using the HP1&3 oscilloscope to remove v i r t u a l l y 100% of the r i p p l e . , ^JUUULV Figure 26: Tuned c i r c u i t to remove high frequency s i g n a l . ^Designed and constructed by the e l e c t r i c a l shop. -123-Araplification of the smaller absorbances was obtained with the use of a pulse amplifier (Keithley model 1 0 9 ) . This a m p l i f i e r i s designed for use i n a 50J~l system and affords a ten times a m p l i f i c a t i o n . I t has a risetime of l e s s than 3 nsec with minimal overshoot. When the pulse ampli f i e r was used i n conjunction with the tuned c i r c u i t only a 5x a m p l i f i c a t i o n could be achieved because i t was necessary to have a $ 0 A load i n the f i l t e r . The ampli-f i e r was powered by a Keithley model 106*1 power supply designed f o r use with the 1 0 9 a m p l i f i e r . The a m p l i f i c a t i o n system was used i n the configuration shown i n Figure 24. The shutter mechanism was also incorporated into the experimental arrangement. When the shutter was opened (commonly fo r 1 / 1 0 0 sec), the e l e c t r i c a l s i g n a l created was converted to a 30V pulse which f i r e d the Febetron. The e l e c t r i c a l "noise" created by the electron beam of the Febetron was used to t r i g g e r , at the convenient mo-ment, the oscilloscope operating i n the single sweep mode. * Designed and constructed by the e l e c t r i c a l workshop -124-3. Results and Discussion Mandelic Acid The decrease i n the concentration of the hydrated electron was monitored by measuring the absorbance at 632.8 nm. The decay curve from the photographs were analyzed using a pseudo f i r s t order plot of In A vs. t over approximately two h a l f - l i v e s . The k i n e t i c p l o t s were examined f o r changes i n slope which might indicate deviations from simple pseudo f i r s t order k i n e t i c s with one rate constant. Typical decay curves and k i n e t i c plots are shown i n Figure 27 . of 1.4 X 10"^. At a concentration of 0.75 M, the hydrogen ion concentration would be s i g n i f i c a n t ^ 10 M. Since the rate constant f o r the reaction of the hydrated electron with hydrogen ions i s 2.3 X 10^M~^sec"^, the electrons would react predominently with the hydrogen ions rather than the a c i d . To prevent t h i s the mandelic a c i d solutions were adjusted to a pH of 9-10 with sodium hydroxide. The f i r s t order k i n e t i c plots f o r both d l - and 1-mandelic acid were reasonably l i n e a r over the f i r s t two h a l f - l i v e s . It was necessary to disregard the f i r s t 20 nsec of the decay because of delayed absorbance. The second order rate constants f o r the two isomers were determined to be Mandelic acid has a d i s s o c i a t i o n constant - 1 2 5 -F i g u r e 27 ( a ) The d e c a y c u r v e o f t h e h y d r a t e d e l e c t r o n i n a 0.551M s o l u t i o n o f 1 - m a n d e l i c a c i d pH 9-10 ( b ) F i r s t - o r d e r k i n e t i c p l o t o f t h e a b o v e d e c a y . -126-Horizontal: 20nsec/div ( a ) V e r t i c a l : lOmV/div time (nsec) - 1 2 7 -dl-mandelic acid 1 . 8 x 1 0 ? M- -^ s e c - ^ 7 - 1 1 1 - mandelic acid 1 . 8 x 1 0 M s e c -No change in slope was observed. The re s u l t s are collected i n Table X. The mandelic acid system i s not an i d e a l one for the observation of the desired e f f e c t . The high concentration of hydroxide ion aggravates the problem of delayed absorbance to the extent that i t was necessary to d i s r e -gard the most promising time region, the f i r s t twenty nsec or so a f t e r the electron pulse. Limited s o l u b i l i t y precludes a hydrated electron l i f e t i m e of ~ 2 0 nsec. The measured rate constant of *>2 x 1 0 7 M""*sec~* pinpoints some of the problem. The hydrated electron reacts too slowly with the mandelate ion. 1-glutamic acid, disodium s a l t : Results s i m i l a r to those f o r mandelic acid were ob-tained i n the pulse r a d i o l y s i s of the disodium s a l t of glutamic acid, COOH (CH 2 ) 2 CH (NH2) COOH, and are collected i n Table X. The solutions were examined at the natural pH of 7.5 and at a concentration of 2 M . A second order rate constant of 6 x 1 0 ° M *sec * was obtained from the analysis of the decay curves. No change i n slope was detected. 7 - 1 - 1 An upper l i m i t of 2: x 1 0 M sec for the reaction of the hydrated electron with 1-glutamic acid at a pH of 5.7 has been reported.^ 2 This rate constant should de-Table X The reation of solvated electrons with c h i r a l molecules Solute Solvent H a l f - l i f e (nsec) M M ^ s e c - 1 number of measurements d l mandelic acid 0.80M pH 9-10 2D HO 2 4 5 1.8 + O.hxlO7 12 1-mandelic acid 0.55M pH 9-10 2D H 20 8 0 1.8± 0.4 x 107 12 1-glutamic acid, disodium s a l t 2.0M, pH; 7 . 5 2D H 20 7 0 6.5+1.0 x 10° 12 d-camphor 7.98x10-3 M 2 . 6 4 x 10"2M 100% EtOH 100%EtOH 5 3 21 1.7* .3 x K T 1.3 + .3 x 109 10 20 1-camphor 8 . 2 3 x 1 0 ~ 3 M 2 . 6 4 x 1 0 ~ 2 M 100%EtOH 100%EtOH ( - ) 3-chlororbutyric a c i d 8 . 5 0 x 1 0 " 2 M 8 . 6 5 x 1 0 " 2 M n-amyl alcohol act. amyl alcohol d l - 3 - c h l o r o b u t y r i c a c i d 6 . 9 8 x 1 0 - 2 M 8 . 6 6 x 1 0 ~ 2 M n-amyl alcohol n-amyl alcohol 5 3 2 3 1 . 7 ± 0 . 3 x 1 0 9 1 . 2 ± 0 . 2 x 1 0 9 1 0 1 0 2 0 3 . 5 ± 0 . 2 x 1 0 s * 1 0 2 0 3 . 6 ± 0 . 2 x 1 0 ° " H 2 8 3 . 6 + 0 . 2 x 1 0 8 1 0 2 6 3 . 8 + 0 . 3 x 1 0 8 1 0 -In-crease as the pH i s increased and the protonated amine-group i s destroyed. Using the disodium s a l t i s equiva-lent to a pH of about 10-11. The rate constant reported i s an upper l i m i t because at the high amino acid concen-t r a t i o n a low l e v e l of reactive impurities may determine the observed l i f e t i m e . Before the next series of o p t i c a l l y active scavengers were studied several modifications to the pulse r a d i o l y s i s system were made i n an attempt to improve the chances of observing the desired k i n e t i c e f f e c t . The changes are det a i l e d i n the experimental section and included a new r a d i o l y s i s c e l l , a pulse a m p l i f i e r , a 500 MHz f i l t e r and a shutter mechanism. A l l were made so as to be able to reduce the dose delivered to the sol u t i o n and thereby reduce the delayed absorbance. Camphor Camphor i s very reactive towards the solvated e l e c -tron because i t contains a carbonyl group. The hydrated Q 1 -1 electron has a rate constant of ^ 10 7M sec with most ketones and aldehydes. To obtain a l i f e t i m e of /~ 20 nsec, _2 the camphor should be /v 2.5 X 10 M. This concentration exceeds the s o l u b i l i t y of camphor i n water, therefore, 100% ethanol was used as the solvent. Solvated electrons -131-are formed r e a d i l y i n ethanol, although the y i e l d i s about 4 0 % l e s s than i n water. ' The absorption spectrum i s very nearly the same as i n water (7\. =730 cf, 715) with max 4 the e x t i n c t i o n c o e f f i c i e n t at t h i s point being 1.5 X 1 0 ' lYT^cm"**. Other than these minor changes, there i s no reason to expect any change i n the basic considerations. Several solutions of both d- and 1 - camphor were i r r a d i a t e d . The data from the various solutions are c o l -lected i n Table X . T y p i c a l l y , traces were obtained using the pulse a m p l i f i e r to give a 5 x a m p l i f i c a t i o n . I 0 was obtained v i s u a l l y without the pulse a m p l i f i e r . I n i t i a l absorbances were generally about 0 . 3 5 , i n d i c a t i n g an i n i -t i a l solvated electron concentration of /v 5 X 1 0 ~ % . The k i n e t i c s should c l e a r l y be pseudo f i r s t order since the camphor concentration was 1 0 0 f o l d greater than e". The decay curves were f i r s t analyzed by f i r s t order k i n e t i c s , the plots being reasonably l i n e a r . To f a c i l i -tate detection of changes i n slope, a computer was then used to analyze the curves assuming the following reaction scheme fo r one of the isomers. e i o l ( r ) + d ^ * p ( m - 6 ' e~ ( 1 ) + d —^2 * P ( I I I - 5 ) s o l The change i n absorbance r e f l e c t s changes in,-the concen-t r a t i o n of the solvated electron. The observed decay was f i t t e d to an equation of the form i - 1 3 2 -j - e i + er*J = e 0 / 2 £e" kl t + e'^ -) ( I I I - I 3 ) where e ( i + r ) ^ s represented by the measured absorbances (as the detection method did not discriminate between the two forms of solvated electron). The f i t was performed by using a non-linear l e a s t squares program from the UBC computer f a c i l i t y - B M D X 8 5 . Both rate constants and the i n i t i a l absorbance were variable parameters. The time was the independent variable and the absorbance was the dependent v a r i a b l e . The program obtained the best f i t to the experimental data by s e l e c t i n g the best rate con-stants. Results for the 7 X 10 M camphor solutions are l i s t e d i n Table X . It was concluded that the decay i s best represented by simple pseudo f i r s t order k i n e t i c s with only one rate constants. The f i r s t order rate con-stants obtained were converted to second order rate con-stants of 1 . 6 8 X 10 9 M~*sec * d-camphor 9 1 - 1 1 . 6 6 X 10 M s e c 1-camphor Electrons i n both of these solutions had h a l f - l i v e s of about 50 nsec. S p i n - f l i p p i n g (e^ < '~ e**r) i s thought to occur at a rate of about 10^ sec"'1' as may be estimated from EPR data. I f t h i s s p i n - f l i p rate i s about the same for solvated electrons i n ethanol as i n water about 5 % might undergo spin inversion i n the two h a l f - l i v e s over which the rate constants were calculated. - 1 3 3 -A second set of s o l u t i o n s of shorter h a l f - l i f e ( ~ 2 5 nsec) was also pulsed. The data were analyzed by the non-linear l e a s t squares program and s i m i l a r r e s u l t s were obtained f o r these 2.64 x lO'^M s o l u t i o n s . Reason-ably l i n e a r f i r s t order p l o t s were obtained over the f i r s t tv/o h a l f - l i v e s . The rate constants f o r both isomers were the same within experimental e r r o r (Table X), although somewhat lower than the values f o r the l e s s concentrated s o l u t i o n s . F i t t i n g the decay to two rate constants gave = &2 about 7 0 # of the time. Chlorobutyric Acid: 3-Chlorobutyric a c i d possesses the c h a r a c t e r i s t i c s of a good scavenger f o r these s t u d i e s . The chloro group i s moderately r e a c t i v e towards the hydrated e l e c t r o n with the —1 — I r a t e constant being of the order of 1 0 8 M~ sec • The solvated e l e c t r o n should undergo about 100 c o l l i s i o n s before r e a c t i n g . The 1-isomer was run i n both a n o n - c h i r a l medium (n-amyl alcohol) and i n a c h i r a l one (act.-amyl a l c o h o l ) . The d l mixture was run as a reference i n n-amyl a l c o h o l . See Figures 28 - 3 0 f o r t y p i c a l decay curves and k i n e t i c p l o t s . The f i r s t order decay of the d l isomer mixture ( 6 . 9 8 x 1 0 M) was l i n e a r over the f i r s t two h a l f - l i v e s . Non-l i n e a r l e a s t squares a n a l y s i s gavethe best f i t as k„ = k^ A A t y p i c a l decay curve of the solvated electron i n an act. amyl alcohol solution which was O.0865M i n (-)-3-chlorobutyric a c i d . -135-Time (nsec) F i g u r e 29 T y p i c a l f i r s t - o r d e r k i n e t i c p l o t s f o r the r e a c t i o n o f the s o l v a t e d e l e c t r o n w i t h ( - ) - 3 - c h l o r o b u t y r i c a c i d . -136-F i g u r e 30 F i r s t o r d e r decay p l o t f o r the s o l v a t e d e l e c t r o n i n an n-amyl a l c o h o l s o l u t i o n o f d l - 3 - c h l o r o b u t y r i c a c i d . I n s e r t : the a c t u a l decay c u r v e . h o r i z o n t a l : 20 n s e c / d i v v e r t i c a l : 10 mV/div HUM 40 Time (nsec) BO -137-75% of the time. The second order rate constant was meas-sured as (3.6 ± 0.3) x 10& M - 1 s e c _ 1 ( average of 15 t r a c e s ) . A more concentrated solutions (8.66 x 10"2M) also resulted i n kj=k2 a majority of the time and with the same second order rate constant. The same r e s u l t s were obtained f o r the 1-isomer i n both the c h i r a l and non-chiral environment with k l = k 2 7 <~^ t^ i e t :*- m e # ^ n e s e c o n d order rate constant was measured as(3.8 ± 0.2) x 10** M - 1sec _ 1. From these r e s u l t s i t was concluded that the reaction of an o p t i c a l l y active solute with solvated electrons could best be described by simple f i r s t order k i n e t i c s . No evidence f o r the existence of species with d i f f e r e n t r e a c t i v i t i e s was found. This was true also i n the case where a c h i r a l medium (act. amyl alcohol) was used, a s i t u a t i o n which might possibly favor the formation of c h i r a l solvated electrons r e s u l t i n g i n {e£ 3 ^ [ e r ] ' I n c a s e s reasonable second order rate constants were obtained, rate constants which were equivalent to those measured f o r the racemic (dl) mixtures when a v a i l a b l e . The method of computer f i t t i n g the decay curves was quite s e n s i t i v e , and would d i f f e r e n t i a t e between rate constants d i f f e r i n g by as l i t t l e as 5%. In most cases the rate constants which gave the best f i t were exactly the same. Thus within the scope of these experiments k^ and k 2 seem to d i f f e r by less than 5%, which means that the -138-s c a v e n g e r s used would o n l y produce a c o n c e n t r a t i o n d i f f e r e n c e i n the e n a n t i o m o r p h i c s o l v a t e d e l e c t r o n s o f 4zl0~7 a t 25nsec. T h i s i s not a s t r i k i n g e f f e c t , and f u r t h e r i n d i c a t e s t h a t these c h i r a l s o l u t e s p r o b a b l y cannot be used to generate a l a r g e enough A c f o r a d i f f e r e n c e i n the a b s o r p t i o n o f r i g h t and l e f t c i r c u l a r l y p o l a r i z e d l i g h t . , t o be detected. .^ W h i l e camphor and the m a n d e l i c and g l u t a m i c a c i d s would not have maximized the d i f f e r e n c e between k^  and kg, c h l o r o b u t y r i c a c i d d i d meet n e a r l y a l l t h e n e c e s s a r y r e q u i r e m e n t s . I t a p p e a r s , t h e r e f o r e , t h a t i f r i g h t - and l e f t - h a n d e d e l e c t r o n s do e x i s t , the r a t e o f t h e i r r e a c t i o n w i t h o p t i c a l i somers must d i f f e r by l e s s t h a n 10%. -139-C. The Interaction of Solvated Electrons with Polarized Light 1. Introduction This section describes an attempt to d i s t i n g u i s h between the postulated l e f t - and right-handed solvated electrons through t h e i r i n t e r a c t i o n s with polarized l i g h t . Two types of p o l a r i z a t i o n are of i n t e r e s t , plane and c i r -cular. Plane polarized l i g h t i s characterized by having i t s e l e c t r i c vector o s c i l l a t i n g i n one plane, while c i r c u l a r l y polarized l i g h t has a r o t a t i n g e l e c t r i c vector. The l i g h t can be either r i g h t or l e f t c i r c u l a r l y polarized depending upon which way (clockwise or counterclockwise) the vector i s turning. It i s often convenient to consider plane polar-ized l i g h t as being composed of equal amounts of r i g h t and l e f t c i r c u l a r y polarized l i g h t (RCP and LCP). Any medium in t e r a c t s with l i g h t with respect to i t s transmission and absorption c h a r a c t e r i s t i c s . C h i r a l mole-cules are distinguished by having d i f f e r e n t transmission c o e f f i c i e n t s (indices of r e f r a c t i o n ) and absorption co-e f f i c i e n t s f o r LCP and RCP l i g h t . I f e i t h e r the r i g h t or l e f t c i r c u l a r l y polarized component of plane polarized l i g h t i s phase s h i f t e d (slowed down) r e l a t i v e to the other, a r o t a t i o n of the plane of p o l a r i z a t i o n occurs. This i s the s i t u a t i o n which p r e v a i l s when polarized l i g h t traverses a c h i r a l medium because the two r e f r a c t i v e indices /| L and/^. R(^ T j and are the r e f r a c t i v e indices for LCP and RCP l i g h t , r e s p e ctively) -HO-are unequal. The phenomenum i s c a l l e d o p t i c a l rotatory power and i s described by the Fresnel Equation .<j> = 7 x ( > ? , . -where * i s the r o t a t i o n i n radians/cm X i s the wavelength i n cm It can be seen from the equation that r e l a t i v e l y small differences i n Y\x and can cause appreciable r o t a t i o n s . For 600 nm l i g h t a r o t a t i o n of $ 0 could occur when £±T\ i s as l i t t l e as ~ 1...7 x 10"" . Right and l e f t c i r c u l a r l y polarized l i g h t are also absorbed d i f f e r e n t l y by a c h i r a l molecule, the molecule having d i f f e r e n t e x t i n c t i o n c o e f f i c i e n t s f o r the two forms of l i g h t . € t = e x t i n c t i o n c o e f f i c i e n t f o r LCP 6rt = e x t i n c t i o n c o e f f i c i e n t f o r RCP Plane polarized l i g h t t raversing a c h i r a l medium becomes e l l i p t i c a l l y polarized as a r e s u l t of the unequal absorption of i t s components. The e l l i p t i c i t y i s given by $ - % ( v ) A compound which absorbs the two c i r c u l a r components un-equally i s said to exhibit c i r c u l a r dichroism. The r e l a t i o n s h i p s between rotatory dispersion (rotatory power as a function of wavelength) and c i r c u l a r dichroism are summarized i n Figure 31. - H l -A F i g u r e 31 The R e l a t i o n s h i p Between O p t i c a l Rotatory-D i s p e r s i o n and C i r c u l a r D i c h r o i s m -142-Several points should be mentioned. Ro t a t o r y d i s p e r s i o n occurs at a l l wavelengths while c i r c u l a r dichroism, an absorption process, occurs over a r e l a t i v e l y narrow range of wavelengths. In general both processes are occurring simultaneously. O p t i c a l rotations, while occurring over a l l wavelengths, are largest near an absorption band. Cir c u l a r dichroism measures the quantity (e^-e^) / ( ( £ T + ^ R ) / 2 ) or A 6 / e w as a function of wavelength. A e i s generally largest at the absorption maximum. Generally i t i s easier to detect C D i n a r e l a t i v e l y weak absorption band (such as a carbonyl n-i^) than i n a strong one because Z^e must be very large f o r C D detection when e A„ i s large. Helicenes are one of the few molecules f o r which a d i f f e r e n t i a l absorption can be measured fo r a strong ab-sorption band. A £ i n the best of cases i s only about 10% of the average extinc t i o n c o e f f i c i e n t . I f r i g h t - and left-handed solvated electrons e x i s t i t i s expected that they would i n t e r a c t d i f f e r e n t l y with the polarized l i g h t forms through d i f f e r e n t r e f r a c t i v e indices and ex t i n c t i o n c o e f f i c i e n t s i n the manner of c h i r a l molecules. Two types of experiments were devised to check these hypotheses. The f i r s t set involved the use of crossed p o l a r i z e r s . I f the analyzing l i g h t i s plane polarized, a p o l a r i z e r placed before the detector w i l l transmit no l i g h t i f i t i s exactly crossed ( 9 0 ° ) with the plane of p o l a r i z a t i o n . - 1 4 3 -However, i f the plane i s s h i f t e d upon traversing a c h i r a l medium ( i . e . , an unequal concentration of r i g h t - and l e f t -handed electrons), some f r a c t i o n of the l i g h t would be transmitted through the crossed p o l a r i z e r . Unequal concentrations of the solvated electrons could develop as a consequence of ( i ) p r e f e r e n t i a l reaction of one form of the solvated electron as discussed e a r l i e r . ( i i ) induced c h i r a l i t y * 8 ? One might expect solvated electrons i n a c h i r a l environment to be c h i r a l themselves since they would be i n a trap formed by either d or 1 solvent molecules. The maximum concentration difference between the enantiomorphic electrons would be ^ 10""^ M, the t o t a l concentration of solvated electrons created. This creation of asymmetry i n a molecule by a c h i r a l environment i s a general phenomenon, and i s termed induced c h i r a l i t y , as opposed to an inherent dissymmetry. The second set of experiments was designed to measure the i n t e r a c t i o n of LCP and RCP with solvated electrons d i r e c t l y . The absorbances of LCP and RCP by the electrons were measured i n both c h i r a l and non-chiral media. In the case of the non-chiral media, [e-(l)J = [e~(r)J and no difference i n the absorption of the two forms of l i g h t should r e s u l t . However, i f i n a c h i r a l medium one of the enantiomorphic solvated electrons were more stable, i t i s possible that [ e - ( l ) J £ (e"*(r|J. In t h i s case, i f z\e were -1LL-large, the absorption of LCP and RCP might not be equal. This second approach depends upon the induction of c h i r a l i t y . Because the electron absorption band, generally thought to be a 2p<^— Is t r a n s i t i o n , i s dependent upon the energy states created by the solvent molecules, i t i s not unreasonable to expect a difference i n the absorption band of the solvated electrons i n a c h i r a l and non-chiral environment. -145-Experimental The materials used were the same as described e a r l i e r (pp. 120-121). The pulse r a d i o l y s i s set-up was s i m i l a r to that used for the k i n e t i c experiments and i s pictured i n Figure 32. The 90° bend was incorporated into the o p t i c a l t r a i n to prevent x-rays from reaching the photomultiplier. (An x-ray s i g n a l might be mistaken for transmission through the crossed p o l a r i z e r . ) The He/Ne laser (Spectra Physics model 120) was plane polarized and was used as the analyzing l i g h t . A polaroid-type p o l a r i z e r i n a calibrated holder was placed before the photomultiplier i n p o s i t i o n A (Figure 32.) ^ When crossed with the la s e r , no transmitted l i g h t was observed when the photomultiplier was operated at moderate voltages (600V). The transmission at various p o l a r i z e r angles was measured (Figure 33 ), and was sinusoidal as expected according to Malus's Equation , • 2 I = k 1^ cos; 0 where I = i n t e n s i t y 1^= incident i n t e n s i t y © = angle k = proportionality constant When the photomultiplier was operated at high voltages (1000V), some l i g h t transmitted by the crossed p o l a r i z e r F i g u r e 32 E x p e r i m e n t a l Set-up f o r P o l a r i z e r E x p e r i m e n t s where L-He/Ne l a s e r B-iwave p l a t e when a p p l i c a b l e . C - R a c i i o l y s i s c e l l F e b e - F e b e t r o n G-Lead s h i e l d i n g I - I r i s t o remove d i f f r a c t i o n r i n g s S - S h u t t e r 0 - O s c i l l o s c o p e T - F i r i n g c i r c u i t f o r F e b e t r o n A - P o l a r i z e r when a p p l i c a b l e E - N e u t r a l D e n s i t y f i l t e r s M - M i r r o r D-Focusing l e n s F - I n t e r f e r e n c e f i l t e r (632.9 nm) P M - P h o t o m u l t i p l i e r PS- Power s u p p l y f o r p h o t o m u l t i p l i e r o • •1 H-N ^ W CD C+ C+ H-3 era O CD era •-j (D CD 01 Fraction of Light Transmitted by P o l a r i z e r • i i o I M O O I O-l •P--o I ro o--Q-+ ro + o-+ o X 0 0 M O K 0) CO-BT o O o 3 B < < ta 03 c+ K CD 03 cr H-O ct CD \ 0 o - A 7 I --148-was detected. The transmission at various angles of the p o l a r i z e r through n-amyl alcohol and act-amyl alcohol and water and a sucrose/water solution were measured and the crossed p o s i t i o n f o r the p o l a r i z e r for each case was determined. With the p o l a r i z e r i n i t s crossed p o s i t i o n , the solutions were pulsed and the signals recorded on f i l m . The photo-graphs were examined for evidence of l i g h t transmission over the f i r s t 100 nsec a f t e r the pulse. The same pro-cedure was followed for the solutions containing solvated electron scavengers.(camphor/EtOH and 3-1 -chlorobutyric a c i d /n-amyl al c o h o l ) . In some preliminary experiments, a 1/4 wave plate (placed i n position B, Figure32' ) was used to convert the plane polarized l i g h t of the laser into c i r c u l a r l y polarized l i g h t . When the plate i s placed at an angle of 45° to a plane of p o l a r i z a t i o n , one of the c i r c u l a r components i s slowed down by a 1/4 of a wavelength and the emergent l i g h t i s either r i g h t or l e f t c i r c u l a r l y polarized. The opposite p o l a r i z a t i o n can be obtained by r o t a t i n g the plate 90°. The experiments consisted of measuring the absorbance of LCP and RCP l i g h t solvated electrons formed i n o p t i c a l l y active solutions (act-amyl-alcohol and sucrose/water) and t h e i r references (amyl alcohol and water, respectively) at 20 and 100 nsec a f t e r the 3 nsec electron pulse. - 1 4 9 -3. Results a « Crossed P o l a r i z e r E x p e r - i m p n t - . c i . P r e f e r e n t i a l formation - The l i g h t transmitted by the po l a r i z e r at various angles f o r act. amyl alcohol was measured f o r two d i f f e r e n t I Q " s (30 and 80 mV). The ro t a t i o n of the plane of p o l a r i z a t i o n due to the alcohol was too small f o r detection with the r e l a t i v e l y i n s e n s i -t i v e arrangement. CoO 2 ^ fo r d-2-methyl-l-butanol i s -4.75° The c e l l i s 0.3 dm long and the density of the alcohol i s 0.83 g/ml. Therefore the expected r o t a t i o n from the alcohol would be 1.2° The same transmission curve was observed for non-active amyl alcohol. No transmission was observed i n either case when the p o l a r i z e r was ex-a c t l y crossed with the plane of p o l a r i z a t i o n . With the p o l a r i z e r exactly crossed the solutions were pulsed, and solvated electrons formed. No transmission was observed over the f i r s t ^150 nsec, i n d i c a t i n g that there was no observable r o t a t i o n of the plane of p o l a r i -zation caused by the solvated electrons. Such r o t a t i o n would be expected i f a preponderance of either r i g h t or left-handed electrons developed as a r e s u l t of the c h i r a l medium. The electrons might have been formed i n a par-t i c u l a r conformation or one of the forms might have been en e r g e t i c a l l y favored and been formed through the spin-f l i p p i n g mechanism. (In the f i r s t A / 1 $ 0 nsec about 1 5 % of the solvated electrons could undergo natural exchange). -150-Th e photomultiplier was operated at high voltages (1000V) f o r increased s e n s i t i v i t y . Similar r e s u l t s were obtained for the water and D-sucrose/water solutions. Rotation of the plane of p o l a r i -zation by the sucrose solution i t s e l f was s i g n i f i c a n t (/^10°). The p o l a r i z e r was adjusted to zero l i g h t trans-mission and so crossed with the plane of r o t a t i o n of the l i g h t a f t e r i t had traversed the s o l u t i o n . No transmission was observed following d e l i v e r y of the electron pulse. i i . P r e f e r e n t i a l Reaction - The second method of ob-t a i n i n g a preponderance of one of the solvated electron isomers i s through the p r e f e r e n t i a l reaction with an op-t i c a l l y active scavenger as discussed e a r l i e r . The scav-...-enger solutions used were d-camphor i n 100% EtOH and 1-3-chlorobutyric acid i n c h i r a l and non-chiral amyl a l c o h o l . In the crossed p o l a r i z e r p o s i t i o n no l i g h t was transmitted before or a f t e r the electron pulse, again i n d i c a t i n g that there was no detectable concentration difference of the l e f t or r i g h t handed electrons. b. Absorption of C i r c u l a r l y Polarized Light The absorption of both r i g h t and l e f t c i r c u l a r l y polarized l i g h t by solvated electrons i n c h i r a l and non-c h i r a l environments was measured at 20 and 100 nsec a f t e r t h e i r formation. In both cases the measured absorbance f o r the c h i r a l environment was i d e n t i c a l to the non--151-c h i r a l case within experimental error. k. Discussion Before the r e s u l t s of the p o l a r i z e r experiments can be evaluated, i t i s necessary to consider the various postulates upon which experimental success i s dependent and the various experimental l i m i t a t i o n s . The fundamental assumption i s that r i g h t - and l e f t -handed electrons ex i s t through e i t h e r an inherent dissymetry or induced c h i r a l i t y . I f t h i s i s true then i t na t u r a l l y follows that they w i l l have d i f f e r e n t r e f r a c t i v e indices and absorption c o e f f i c i e n t s f o r LCP and RCPe The success of the experiments, however, depends upon creating an unequal concentration of the two isomers. It i s postulated that t h i s can be accomplished by either induction by a c h i r a l medium or by p r e f e r e n t i a l reaction of one of the isomers with a c h i r a l scavenger. The e a r l i e r k i n e t i c experiments indicate that k_^  ar.d )&2 do not d i f f e r by as much as 10%, and probably by not more than 5% i f at a l l . The concentration d i f f e r e n t i a l that might develop would be only be ^  10 'M. Induced c h i r a l i t y i s the other possible mechanism for obtaining unequal concentrations. In t h i s case 10 M represents an upper bound to the difference that might exist by assuming that a l l solvated electrons formed i n a c h i r a l environment are c h i r a l themselves. -152-I f a concentration difference of 1 x 10~^M can develop, i t i s i n t e r e s t i n g to see what magnitude ofA i \ = 1^  - t i s necessary f o r a detectable s i g n a l i n the crossed p o l a r i z e r experiment. Assumung an I Q of 200 mV, a 5 ° r o t a t i o n of the plane of p o l a r i z a t i o n i s necessary to obtain a 2mV si g n a l which i s the minimum si g n a l required f o r a d e f i n i t e i n d i c a t i o n of transmission through the p o l a r i z e r . According to the Fresnel equation a 5^ r o t a t i o n requires a A)| of Q 1.8 x 10" . Even the maximum concentration difference of - 5 1 x 10 M would require an extremely large difference between 1^ and 1 f o r the solvated electron before a r o t a t i o n could be detected. WhileAi\does increase i n the v i c i n i t y of an absorption band (Figure 3 D , i t i s u n l i k e l y that the necessary difference could be r e a l i z e d . There i s even le s s l i k e l i h o o d of an e f f e c t i f the concentration difference i s dependent upon d i f f e r e n t reaction rates. Thus while i n general small differences i n r e f r a c t i v e indices r e s u l t i n reasonable r o t a t i o n s , the required r o t a t i o n i s too large and the concentration too small f o r there to be transmission through the p o l a r i z e r . A s i m i l a r analysis for the absorption of LCP and RCP l i g h t can be done. I f a A A could be detected, and i f the difference i n concentration of the enantiomorphic solvated electrons were 1 x 10 , a A€ of 2000 would be required f o r a p o s i t i v e r e s u l t . Since no difference i n absorption was detected, the asymmetry fac t o r g, where g SS(^JJ - e R V ( G i . + eR^ i s -^ ° » o 8» It i s not s u r p r i s i n g that no p o s i t i v e r e s u l t i s seen as i n general A £ i s l e s s than 10% of (€ t + <=^ )/2. -153-For the experiments to succeed i t would be necessary to have e i t h e r a large concentration d i f f e r e n t i a l or a more sensi t i v e detection system. Further experiments along t h i s l i n e have been performed recently i n t h i s laboratory by Dr. R. May. The CD of frozen solutions of trapped electrons i n c h i r a l and non-chiral media was measured using a JASCO ORD/CD 5 spectrometer. In a further set of experiments trapped electrons were bleached with l i g h t from a polarized ruby l a s e r . The absorption at both 633 and 694 nm was monitored absolutely as well as induced transmission through crossed p o l a r i z e r s . The b l e a c h i n g experiments might allow a larger concentration d i f f e r e n t i a l to be created through successive excitations with either LCP or RCP. The normal CD experiment on the JASCO i s the inherently most s e n s i t i v e method. Again no evidence f o r the existence of c h i r a l trapped electrons was obtained. -154-5. Conclusions and Suggestion? for Further Study Evidence f o r the existence of enantiomorphic solvated electrons (right and l e f t handed electrons) was not ob-tained from either the observation of differences i n re-action rates with o p t i c a l isomers or differences i n t h e i r i n t e r a c t i o n with polarized l i g h t . The l a t t e r i s inherently the better approach because c h i r a l molecules are character-ized by d i f f e r e n t r e f r a c t i v e indices and absorption coer-f f i c i e n t s f o r RCP and LCP l i g h t . Unfortunately, the ex-perimental l i m i t a t i o n s on these measurements were severe. On the other hand, d i f f e r e n t reaction rates might or might not characterize the reaction of enantiomorphic solvated electrons with c h i r a l molecules. For the isomers measured such a difference i n rate constants must be l e s s than 5%, however enantiomorphic might exist which would exhibit a greater d i f f e r e n c e . (A h e l i c a l compound such as a sub-s t i t u t e d helicene with a large asymmetry might be more selective-}. There are several d i r e c t i o n s further experiments concerning t h i s problem might take, but the best approach would be to t r y to obtain solvated electrons of predom-in a n t l y one-handedness. Then the i n t e r a c t i o n with p a i r s of o p t i c a l isomers and LCP and RCP could be measured d i r e c t l y . Experimentally t h i s separation might be accom-plished by passing the electron beam through a magnetic f i e l d thus separating the two spin s t a t e s . -155-It might also be possible to use the Cerenkov l i g h t produced by the electron beam as a probe f o r o p t i c a l a c t i v i t y . Cerenkov l i g h t produced by a 0.5 MeV electron beam i s l i n e a r l y polarized. The inherent p o l a r i z a t i o n of the Cerenkov l i g h t could be used to investigate r a d i a t i o n produced asymmetry on the subnanosecond time scale. However, such experiments would be t e c h n i c a l l y very d i f f i c u l t . Along s i m i l a r l i n e s , the photochemical production of solvated electrons using c i r c u l a r l y polarized l i g h t could be investigated. I t would be possible to measure and compare the y i e l d and. r e a c t i v i t y of solvated electrons formed by the polarized l i g h t i n c h i r a l and non-chiral solvents. Investigation into the c h i r a l i t y of the solvated electron, although unproductive at present, would seem to be worth pursuing because of the implications i t might have on the creation of the asymmetry found on earth. -156-REFERENCES 1 . a) J . W. T. Spinks and R. J . Wood, An Introduction to Radiation Chemistry, New York, John Wiley and Son, b) E. J . Henley and E. R. Johnson, The Chemistry and  Physics of High Energy Reactions t Washington, University Press, 1 9 o 9 . c) James H. O'Donnell and David F. Sangster, PrincipT.es  of Radiation Chemistry, New York, American E l e s e v i e r Publishing Co., 1 9 7 0 . 2 . A. Mozumder and J . L. Magee, Radiat. Res., 2 8 , 2 0 3 ( 1 9 6 6 ) . 3 . a) J . K. Thomas i n Adv. Rad. Chem., 1 , edited by M. Burton and J . L. Magee, Toronto, Wiley-Interscience, 1 9 6 9 , pp. 103-199. b) I. G. Draganic and Z. D. Draganic, The Radiation Chemistfcy of Water, New York, N.Y., Academic Press, 1 9 7 1 . c) A. K. Pikaev, Pulse Radiolysis of Water and Aqueous Solutions, edited by E. J . Hart f o r t r a n s l a t i o n , Bloomington,USA, Indiana University Press, 1 9 6 7 . 4. aO R. L. Platzman i n Radiation Biology and Medicine, ed. W. D. Claus, New York, Addison Wesley, 1956", p. 1 5 . b) A. Kuppermann, J . Chem. E d u c , J 3 6 , 2 7 9 ( 1 9 5 9 ) . 5 . 'W.H. Hamill, J . Phys. Chem., 211 1341 ( 1969 ' ) ' . a) A. H. Samuel and . L. Magee, J . Chem. Phys., 2 1 , 1080 ( 1 9 5 3 ) . ~ ~ b) J . L. Magee, Radiat. Res. Suppl., Z^ , 2 0 ( 1 9 6 4 ) . . A. K. Pikaev, The Solvated Electron i n Radiation Chemistry, translated by J. Schmorak~7 Israel' Program f o r S c i e n t i f i c Translation, Jerusalem, 1 9 7 1 » PP. 1 2 - 1 4 . (Available from U.S. Dept. of Commerce.). . a) R. L. Platzman, Physical and Chemical Aspects of Basic Mechanisms i n Radiobiology. edited by J . L . Magee,et a l . , Washington, U. S. Natl. Acad. S c i . Publ. no. 3 0 5 , 3 4 , ( 1 9 5 3 ) . - 1 5 7 -b) H. FrBhlich and R. L. Platzman, Phys. Rev., 9 2 , 1152 (1953) . ~ ~ c) D. E. Lea, Actions of Radiation on Liv i n g C e l l s , 2 Ed., Cambridge, University Press, l9b2. d) L. M. Gray, J. Chem. Phys., 172(1951) . 9 . E. J . Hart, S . Gordon and E. M. Fielden, J. Phys. Chem., 2 0 , 1 5 0 ( 1 9 6 6 ) . 1 0 . a) Edwin J. Hart and Michael Anbar, The Hydrated Electron. New York, Wiley-Interscience, 1970. b) D. C. Walker, Quart. Rev., 2 1 , 79(1967) . 1 1 . R . S c h i l l e r , J. Chem. Phys., 2278(1967). 12 . G. Freeman and J . M. Fayadh, j ; Chem. Phys., 86(1965) . 1 3 . J. Jortner and S: A. Rice, Adv. Chem. Ser., _5_0, 1 (1965) . 1 4 . E. M. Fielden and E. J. Hart, Trans. Faraday S o c , 63 . 2975(1967). 1 5 . See, f o r example, Geraldine A. Kenney and David C. Walker, "Hydrated Electrons and Electrochemistry", i n E l e c t r o a n a l y t i c a l Chemistry, 1971, pp. 4 4 - 5 0 . 1 6 . W. C. Gottschall and E. J. Hart, J. Phys. Chem., 7 1 . 2102 (1967) . 1 7 . U. Schindewolf, H. Kohrmann and G. Lang, Angew. Chem., I n t l , Ed., 8 , 512(1969). 1 8 . C. Avery, J . R. Remko and B. Smaller, J . Chem. Phys., 49 . 951(1968) . ' » azf 1 9 . a) Z. D. Dragjnic and I . G. ^raganic, J . Phys. Chem., 76 . 2733(1972). b) Z. D. Draganic and I . G. Draganic, J. Phys. Chem., 7 5 , 3950(1971). 20 . A. Kuppermann in Radiation Research, edited by G. S i l i n i , Amsterdam, North Holland Publishing Co., I967 . 2 1 . a:) H. A. Schwarz, J. Phys. Chem., 21$ 1 9 2 8 ( 1 9 6 9 ) . b) H. A. Schwarz, J. Chem. Phys., 5_5_, 3647(1971). 22 . G. R. Freeman, J. Chem. Phys., ^ 6 , 2822(1967). -158-2 3 . J. C. Russell and G. R. Freeman, J. Chem. Phys., 4 8 , 9 0 ( 1 9 6 8 ) . 2 4 . W. H. Hamill, J . Phys. Chem., 21, 1 3 4 1(1969). 2 5 . M. J . B r o n s k i l l . R. K. Wolff and J . W. Hunt, J . Phys. Chem., 21. 1 1 7 5(1969). 26. M. J . B r o n s k i l l , R. K. Wolff and J, W. Hunt, J . Chem. Phys., £1, 4 2 0 1(1970). 27. J . E. Al d r i c h , M. J. B r o n s k i l l , R. K. Wolff and J . W. Hunt, J . Chem. Phys., £5_, 530(1971). 28. R. K. Wolff, M. J. B r o n s k i l l and J . W. Hunt, J. Chem. Phys., 11, 4 2 1 1 ( 1 9 7 0 ) . 2 9 . John W. Hunt, R. K. Wolff, M. J . B r o n s k i l l , Charles D. Jonah, E. J . Hart and Max S. Matheson, J . Phys. Chem., 22, 4 2 5 < 1 9 7 3 ) . 3 0 . R. K. Wolff, M. J. B r o n s k i l l , J. E. Al d r i c h and J. W. Hunt, J . Phys. Chem., 21, 1 3 5 0(1973)., 3 1 . Samuel Magee i n Summary of Proceedings of the Fourth Informal Conference on Radiation Chemistry of Water, University of Notre Dame, March 1 9 6 1 , p. 62. 3 2 . C. D. Jonah, E. J . Hart and M. S. Matheson, J. Phys. Chem., 2Z, 1 8 3 8 ( 1 9 7 3 ) . 3 3 . J . W. Hunt and J . K. Thomas, Radiat. Res., ^ 2 , 1 4 9 ( 1 9 6 7 ) . 3 4 . J. K. Thomas and R. V. Bensasson, J. Chem. Phys., 4 6 , 4 1 4 7 ( 1 9 6 7 ) . 3 5 . G. V. Buxton, F. C. R. C a t t e l l and F. S. Dainton, Trans. Faraday S o c , 62, 687(1971); Chem. Commun. , 1971 , 2 7 . 3 6 . G.V. Buxton, Proc. R. Soc. London.A., _3_28, 9 ( 1 9 7 2 ) . 3 7 . a) T. Sawai and W. H. Hamill, J . Phys. Chem.; 2k, 3 9 1 4 ( 1 9 7 0 ) . b) S. Khorana and W. H. Hamill, J . Phys. Chem., 7 5 , 3 0 8 1 ( 1 9 7 1 ) . 3 8 . F. S. Dainton and W. S. Watt, Nature, 1£5_, 1 2 9 4(1962). 3 9 . S. C. Wallace, Ph. D. Thesis, The University of B r i t i s h Columbia, 1971. - 1 5 9 -40. G. h. Kenney-'vVc- 11a oe, E. A . Sh&ede, D. C. W a l k e r and S. C. W a l l a c e , I n t . J . l t e d i a t . Phys. Chem., 4, 209(1972). 41.. S. C. W a l l a c e and D. C. W a l k e r , J . Phys. Chem., 76, 3780(1972). " ~ 42. J . V. J e l l e y , Cerenkov R a d i a t i o n , New Y o r k , Pergamon P r e s s , 1958. 43. D. C. Walker and S. C. W a l l a c e , Chem. Phys. L e t t e r s , 6,111 (1970). 44. F i e l d E m i s s i o n D a t a . 45. F i e l d E m i s s i o n C o r p o r a t i o n , M c M i n n v i l l e , Oregon, USA. 46. F. M. C a r b o n n i e r of F i e l d E m i s s i o n , p r i v a t e communication. 47. M a n u f a c t u r e r s o f the F e b e t r o n , F i e l d E m i s s i o n C o r p o r a t i o n . 48. a) C. W i l l i s , A. V/. Boyd and 0. A. M i l l e r , R a d i a t . Res., 46, 428(1971). b) J . McDonald, A. P i n k e r t o n , H. W e i s s and E. R. Epp, R a d i a t . Res., 49, 495(1972). c) R e f e r e n c e 40. d) C. W i l l i s , 0 . A. M i l l e r , A . E. R o t h w e l l and A. W. Boyd, R a d i a t . Res., 35, 428(1968). 49. a) L. C. Spencer, N.B.S.(U.S.) Monograph, 1, 1959. b) L. C. Spen c e r , Phys. Rev., 96t 1597(1955). c) Ann T. Helms, N.B.S.(U.S.) C i r c u l a r , 577, 1956. ' 5 9 . a) Mahlman and T. J . S w o r s k i i n The C h e m i s t r y o f I o n i z a t i o n  and E x c i t a t i o n , e d i t e d by G. R. A. Johnson and G. S c h o l e s , London, T a y l o r end F r a n c i s , L t d . , 1 9 6 7 , p. 2 5 9 • 5 1 . Gideon C z a p s k i and Emanuel P e l e d , J . Phys. Chem., 7 7 , 3 6 2 6 ( 1 9 7 1 ) . 5 2 . 0. F. M i c i c , V . M a r k o v i c and D. N i k o l i c , J . Phys. Chem., 7 7 , 2 5 2 7 ( 1 9 7 3 ) . 5 3 . Z. D. D r a g a n i c ?nd I.G. D r a g a n i c , J . Phys. Chem., 7 7 , 2 6 9 1 ( 1 9 7 3 ) . 5 4 . Gideon C z a p s k i ^nd Emanuel P e l e d , J . Phys. Chem., 7 7 , 8 9 3 ( 1 9 7 3 ) . -160-55. J . P . K e n n e , R a d i a t . R e s . , 22, 1(1964). 56. a ) V.'. C . G o t t s c h a l l a n d E . J . H a r t , J . P h y s . C h e m . , 71, 2102(1967). b ) E . M . F i e l d e n a n d E . J . H a r t , R a d i a t . R e s . , 32, 564 (1967). ~ 57. M . A n b a r a n d E . J . H a r t , J . P h y s . C h e m . , 69, 1244(1965). 58. T . I . B a l k a s , J . H . F e n d l e r a n d R . H . S c h u l e r , J . P h y s . C h e m . , 74, 4497(1970). 59. S t e f a n J . R z a d a n d R o b e r t H . S c h u l e r , J . P h y s . C h e m . , 77, 1926(1973). ~"" 60. P . M . R e n t z e p i s , R . P . J o n e s a n d J . J o r t n e r , C h e m . P h y s . L e t t e r s , l j j , 480(1972). 61. L . G i l l e s , J . E . A l d r i c h a n d J . V/. H u n t , N a t u r e , 243, 70 (1973). 62. a ) C y r i l P o n n a m p e r u n a , T h e O r i g i n o f L i f e , New Y o r k , E . P . B u t t o n , 1972, p p . 105-113. b ) L . E . O r g e l , The O r i g i n o f L i f e : M o l e c u l e s a n d N a t u r a l  S e l e c t i o n . New Y o r k , W i l e y , ~ T 9 7 3 , p p . 155-1*5BT 63. G . M . S c h w a b a n d L . R u l d o p h , N a t u r w i s s . , 20, 363(1932). 64. W . T h i e m a n n a n d K . W a g e n e r , A n g e w , C h e m . I n t e r n . E d . , 9, 740(1970). 65. I . v a n ' t H o f f , L a g e r u n g d e r A t o m e i m R a u m e , B r a u n s c h w e i g , 1894. 66. W . K u h n a n d E . K n o p f , N a t u r w i s s . , 18, 183(1930). 67. a ) T . L . D a v i e s a n d R . R e g g i e , J . A m e r . C h e m . S o c , 57, 377, 1622 (1935). ~~ b ) R e v i e w e d i n T . L . V. U l b r i c h t , Q u a r t . R e v . C h e m . S o c , 13, 48(1959). 68. a ) W i l l i a m A . B e r n s t e i n , M e l v i n C a l v i n a n d O l e B u c h a r d t , J . A i r i e r . C h e m . S o c , 94, 494(1972). b ) A . M o r a d p o u r , J . F . N i c o u l d , G . B a l a v o i n e , H , K a g a n a n d G . T s o u c a n s , J . A m e r . C h e m . S o c , 93, 2353(1971). - 1 6 1 -6 9 . T. D. Lee and C-N- Yang, Phys. Rev., 104, 2 5 4 ( 1 9 5 6 ) . 7 0 . C. S. Wu, E. Ambler, R. W. Haywood, D- D• Hopper, and R. P. Hudson, Phys. Rev., 1 0 5 , 1 4 1 3 ( 1 9 5 7 ) . 7 1 . T. D. Lee and C. N. Yang, Phys. Rev., 1 0 5 , 1671(19571. 7 2 . a) M. Goldhaber, L. G r o d z l n s and A. W. Sa n y a r , Phys. Rev., 106, 8 2 6 ( 1 9 5 7 ) . 7 b) 3 . G a l s t e r and H. Schopper, Phys. Rev. L e t t e r s , 1, 3 3 0 ( 1 9 5 8 ) . c) A. B i s I s ami L. Zappa, Phys. Rev. L e t t e r s , 1 , 3 3 2 ( 1 9 5 8 ) . 7 3 . P. Boehm and A. H. Wapstra, Phys. Rev., 1 0 6 , 1 3 6 4 ( 1 9 5 7 ) , 1 0 7 , 1 2 0 2 , 1 4 6 2 ( 1 9 5 7 ) . 7 4 . T. L. V. U l b r i c h t and F. V e s t e r , T e t r a h e d r o n , 1 8 , 6 2 9 ( 1 9 6 2 ) . 7 5 . A . S . Garay, N a t u r e , 2 1 9 , 3 3 8 ( 1 9 6 8 ) . 76." A. S. Garay, L. K e s z t h e l y i , I . Demeter and P. H r a s k o , Chem. Phys. L e t t . , 2 3 , 5 4 9 ( 1 9 7 3 ) . 7 7 . A l e c L i u , Honours T h e s i s , The U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1 9 6 8 . 7 8 . a) H. Schopper, P h i l . Mag., 2 , 7 1 0 ( 1 9 5 7 ) . b ) . K. A l d e r , B. S t e c h and A. W i n t h e r , Phys. Rev., 1 0 7 , 7 2 8 ( 1 9 5 7 ) . 79 - G e r a l d i n e A. Kenney, Ph. D. T h e s i s , The U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1 9 7 0 . 80 . G>. A. Kenney and D. C. W a l k e r , J . Chem. Phys., 5 3 , 1 2 8 2 ( 1 9 7 0 ) . 8 1 . G. A. Kenney and D. C. W a l k e r , J . Chem. Phys., 5 0 , 4074 ( 1 9 6 9 ) . 8 2 . R. Braams, R a d i a t . Res., 2 7 , 3 1 9 ( 1 9 6 6 ) . 8 3 - A. Taub, D. A. H a r t e r , M. C. Sauer, J r . and L. M. Dorfman, J . Chem. Phys., 4JL, 9 7 9 ( 1 9 6 4 ) . 84. M. C. Sauer, S. A r a l and L. M. Dorfman, J . Chem Phys., 42, 7 0 8 ( 1 9 6 5 ) . ~ ~ -162-85. aO W. A. S c h u r c l i f f , Polarized Light: Production and Use, Cambridge, Ma., Harvard University Press, 1962. b) D. Clark and J . F. Grainger, Polarized Light and Optical  Measurement, Toronto, Pergamon Press, 1971. 86. W. Kuhn, Trans. Faraday S o c , &6, 293(1930) as quoted i n C. Djerassi, Optical Roratory Dispersion. New York, McGraw H i l l , I960. 87. a) A. Bosnich, J . Amer. Chem. S o c , 89, 6143(1967). b) L. D. Hayward and R. N. Totty, Chem. Commun., 676(1969). c) c E . Axelrod, G. Barth and E. Bunnenberg, Tetrahedron Lett., 5031(1969). d) L. D. Hayward and R. N. Totty, Can. J. Chem., 49, 624 88. R. May, 5 International Congress of Radiation Research. Seattle, 1974. 89. A. Hannes, Z. Physik, 186, 277(1965). 

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